Well, i can see ThunderGr's point here but i don't think i can absolutely agree with him. Tank performance as a whole depends a lot on experience of it's crew. One wrong order from commander, one mistake from driver, poor reaction of gunner and tank can be out of combat or destroyed. Yeap, modern systems do a lot of work but it's just assistance. All actions and decisions are still made by humans. So manned vehicles can level up and increase their stats in my opinion.
But anyway, the main problem with vehicles in X-Com series i see in that developers trying to put em in two roles at the same time: scouts and fire support. But this two are totally opposite things. For fire support you want something with massive firepower. That means it will be large in size and probably manned. Because of size it became a good target so you want to protect it. You can do it either by armour to withstand some punishment or by range (if it is far from enemy - he can't damage it). Because in our case range isn't an option armour is the only possible solution and that's basically your MBT (Main Battle Tank) description. As for scout you want something fast and agile to run away if needed. Also it must be small so it'll be harder to spot and destroy. That most likely make it unarmed (to save space). And probably you want it unmanned in case it will be destroyed. For that you have all that little remote controlled scout planes and helicopters in modern army.
So while X-Com and Xenonauts makes vehicles look like MBTs (large target, good firepower, decent armour) they want you to use it as a scout. But WW2 already told everyone that tanks in reach of infantry is an easiest prey (urban combat with all that soldiers armed with panzerschrecks). And like at WW2 times some poor lads have to take point and scout ahead. That's the first thing.
Second one about weapons. Tanks don't use sabot or HEAT munition against infantry. Good old HE rounds. Because you don't need an insane penetration rate to kill a rifleman. Also it's kind of hard to hit him with that rounds. While HE provides splash damage so you don't need to actually hit that rifleman - just hit something solid nearby. I'm disappointed in mounted machine guns realisation (more about it further) so only vehicle with rockets somehow feat that fire support role in my opinion.
So in my opinion you can still have use for vehicles without levelling it up. Just make em feat their roles. For example make a small unarmed RC buggy (1 spot in transport) with low hp and armour (1 hit equals death) but with good sight and a good amount of TUs. Make it cheap and fast to manufacture (or just purchase as an ordinary weapon). So we get ourself a scout that feats this role!
As for big fire support vehicles i can only suggest re-think the whole idea behind .50 cals, lasers and other weapons. Basically idea of machine gun type weapons is in throwing as much metal or energy in enemy general direction in short period of time as possible (moar dakka!!!). But currently M2 bursts are very short (3 short from each barrel of twin M2 Browning as far as i remember - half of a standard LMG burst), munition is very limited, accuracy is very low so it reminds me more shooting AT rounds from tank in advancing infantry. I said about dakka already and 72 bullets for main weapon of armoured car? That's a joke. As for accuracy some may know, some may not but scoped M2 was used as a sniper f***ing rifle during Korean and Vietnam Wars. Ok, that was in semi-automatic mode (some modification involved) while in full auto it is far less accurate but that 60 degree cone of fire it usually shows in game is absurd.
So while twin M2 Hunter is overall better than guy with a rifle (at the beginning at least) it takes 4 slots in transport. And that's it. Four guys with rifles are usually better and versatile than a single twin M2 Hunter.