VTOL troops are a great challenge to use properly, mostly because they lose almost all of the advantages of more conventional infantry, gain more disadvantages, and don't appear to gain all that many advantages compared to using actual VTOLs.
Their main disadvantage lies in their VTOL movement. Specifically, that they have the same terrain restrictions as actual VTOLs. They can't enter buildings, can't enter woods, and hovering in midair is pretty much the definition of in the open, so any time a platoon gets hit, they're treated as being out of cover for purposes of taking damage. Worse, they're airborne most of the time, so in addition to being vulnerable to anti-infantry weapons, anti-air ones are effective against them too.
The next disadvantage is that because they're zipping around in such tiny vehicles, they don't have the ability to carry more than one support weapon per squad, so your firepower and range limits are much lower than most other platoon types. Also means that since you can't achieve the magic two-supports-per-squad, a lot of support weapon special abilities(burst damage, IDF, TAG, etc) are forever denied you.
That being said, they do have advantages, again, mostly tied to their movement profile. VTOL troops are FAST. It may not look like much compared to most mechs or tanks(to say nothing about regular VTOLs), but for infantry, 5MP sustained is insane, to say nothing of their ability to simply zip over most terrain that other infantry must detour around. Unlike APC-borne troops who must spend turns loading and unloading from their transports, anything VTOL troops can do, they can do on the move. While limited by the range of their primary weapons, there are still pretty long-ranged rifles out there, which combined with their slurred makes these guys well-suited for harassing other infantry(and even some BA) that might be limited to MG-style ranges.
Jim 1701 mentioned their utility as spotters. While they cannot TAG, and while ghillie suits and rotor backpacks(or smartcar-sized minichoppers) are pretty mutually exclusive, they are very good at calling in indirect fire because despite their speed, they still don't add any attacker movement mods to a shot. They can spot on the move(and thus always moving to where you need them most), or hover above a hill and spot all day, ducking behind cover whenever it looks like they've earned attention from enemy gunners. Also, their ability to hover high above the ground makes it harder to quickly kill them with artillery or airstrikes, since they can fly higher than an artillery shell's or bomb's blast area.
Finally, there's those other roles for which raw damage has little or no bearing, most notably combat engineers. Most of the tasks engineers preform require a number of turns to achieve, so the sooner you get to the target area, the better, and VTOL troops can definitely get almost anywhere a lot sooner than most. That being said, I find the mental image of VTOL bridgebuilder troops to be hilarious. :)
