"(So) the purpose of a sentinel is to carry what an infantry team cannot into places where conventional vehicles cannot go. That's it. In any other situation, the resources involved in keeping a walking machine working make them not worth it."
Not a discussion about lists and PV. But as it's me, you knew that.
The Imperial Guard traditionally has a number of options for recce troops. So we'll have a look and see if we can figure out why the elephant in the room (the walker in the motor pool) even exists.
Foot soldiers. Specialised infantry assets have always been used in recce. Eric Bana in Backhawk Down, Space Marine Scouts on the table top, even down to the 'point man' at the front of his platoon. Recently in the British Army this has been either a bloke with a metal detector/explosive sniffer or a bomb finding dog handler and his furry asset. I Vietnam War films, at least, it's a guy on full auto with the safety off. For more fortykay-ness, try reading anything about the Tanith First and Only. They're a recce unit.
Foot recce are good up mountains, in swamps and jungles, in buildings, over rubble, down tunnels, on trains and just about anywhere else you can think of. From a campaigning point of view, they can be strategically slow, as they tend to walk everywhere. But they use the same small arms ammunition as everybody else, so logistics arn't a vast problem.
Salamander. The armoured formation option. Open topped, loaded with STAR kit, overcharged engine, pretty well fit for purpose. Within an armoured formation there is a robust supply chain keeping the salamander fleet fuelled up and working. The low profile makes it easy (for an armoured vehicle) to hide and its armour and the fact that it cannot fall over make it the most durable of all of the recce options.
On the subject of Chimera chassis vehicles, I'd like to moot the idea of an M3 (CAV) Chimera. For those unaware, there is the M2 (Infantry) Bradley that most people think of, which carries an infantry section. Like the Chimera. Then there is the M3 version which has a smaller crew (3 or 4) and carries more ATGW missiles and has a different battlefield role. I recommend "Carnivore" by the gloriously named Dillard Johnson for a decent picture. Any way, there could be CAV Chimera, as a recce asset. If you only to convert one or two mid turret Chimera, here's your excuse.
One of the main advantages here is fire power, whether autocannon, multi laser, heavy stubber, bolter, HK Missile(s) or whatever, there are usually options to get more rounds down the range. Those two are going to work best in places where the other armoured assets shine; ie not up mountains, not in swamps, not in close terrain like dense urban areas.
Rough Riders. Including the Death Riders. The reasoning must go something like; "If we can send human soldiers, we can send horses. And if we have have to import all of the human food, we can import the horse fodder as well." Better yet, if the planet has been terraformed, the horses can eat grass. One huge chunk of the supply chain has just been circumvented. And these beasties need farriers and veterinarians, not spare engines and endless replacement track links, a half a dozen people for a small battalion sized unit, not thousands of tonnes of spares, jigs, rigs, tools and plant items. If you want efficiency in the Departmento Munitorium, then rough riders are it. No-brainer, as Kevin Bacon would say.
These can handle very rough terrain and in a charge, as a bunch, could be devastating. Although following Napoleonic trends in Europe, only one cavalry regiment began the US Civil War with lances and reputedly threw most of them away in their first engagement. They should've taken the explosive tipped ones...
Technicals - 4x4s mounting a crew served weapon. Cheap, easy to source and useful over terrain that is mostly flat, including built up areas. There are two ways to go here; import your own, reliable, trained on, fit or purpose vehicle which you know is available because you brought it with you. The Tauros Option or WMIK, if you're a Brit. The other, which is not modelled in 40K AFAIK, is to commandeer locally produced "Toyota pick ups" and bolt your squad support weapons to them. I suspect that as there would be a ready market for utility vehicles on every inhabited world, that the DM would possibly be set up to use this option (carrying weapon mounting kits and green paint etc).
These offer speed and firepower. Being recce troops, you'd hope that they would run away if they actually came into contact with the enemy, so a lack of armour shouldn't be a problem. The other 'win' for using locally sourced vehicles is availability of spares, trained and experienced mechanics (assuming there are any left) and indeed, possibly local manufacturing facilities to make new (military specific) ones. If it is a world lacking in resources, where fuel for the armoured fleet has to be imported, you can bet your bottom dollar that the locals run their utility trucks on spomk juice or what ever is locally available - another bit of pressure, however tiny, taken off of the interstellar supply chain.
And that brings us to the one with no real world picture. Oh all right then.
Sentinels. Not for closed terrain, not for anywhere you can drive, certainly not for anywhere you can take a tank. Not for somewhere too wet. Not for open terrain. Not for lumber yards. I would estimate that the support team and resources required to keep one Sentinel in the field are at least equal to those for a Main Battle Tank, probably greater - a walker is a vastly more complex machine, requiring an extremely high power to weight ratio. Looking at the Sentinel model and the size of it's engine, we might reasonably assume that it revs much higher than conventional vehicle engines to provide the required power. So it uses more lubricants and wears through its working parts quicker. All of it's systems are like this. The hydraulics for its legs are complex, vulnerable and would require a much higher maintenance schedule than the other recce options discussed here.
If this is the recce asset of choice for picking its way through an ice or lava floe, then it's 'walking brain/balance tubes' will be phenomenally important and quite likely require re-calibration on an almost constant basis. And once the Sentinel falls over, it needs a crane to get back up again.
The maintenance team is bigger than that of an MBT, and possibly higher skilled and definitely uses more resources, as discussed.
So WTF is it for ? It is for use somewhere that requires heavy weapons to be moved around faster than the infantry types can manage themselves. So somewhere like moderately dense urban or jungle (urban jungle ! See what I did there ?). Somewhere were a heavy weapon is likely to have to break cover in order to fire effectively and therefore needs to be slightly more durable than an infantryman. The other thing that these are is impressive. If the AT-STs had been playing their heavy flamers over the ewoks, the rebels would never have been able to take the shield generator station. If you're a section on foot and one of these sudden looms out of the rubble at you, you'd probably want to be somewhere else.
Should the scouting rule be limited to the cabriolet version ? No. the armoured one is also, as I have described, a light infantry tool. It might not be doing the recce itself, but is there to give the recce assets some much needed punch, as they tend to lack firepower. The cabriolet may be used in a recce role itself, but it is a poor option for finding out what is waiting around that corner - compared to a make up mirror on the end of a bent stick - too fragile to be as useful as the armoured one and yet still requiring resources that might keep an MBT in the field for a year, just to keep it walking around for a fortnight.
Of course, the rules remove the scout move from the armoured one, but there you go; the rules are written by games designers, not people who think about military hardware.