On external missions, you're limited by your dropship size. Since a dropship can carry at best 8 soldiers (alright, more in a later stage of game development. There are plans to increase the soldier limit. Here's to hoping it won't go past 12), that means you will typically never bring more than 2 UGVs on a mission anyway, as one UGV takes up the space of 4 soldiers. Some more advanced dropships could arguably hold more if they were designed for UGV transport, but even so each UGV would take up one soldier slot on the HUD, and they're not very versatile in close quarters, which is a good reason not to bring more than a couple.
So in effect, such a limitation would only really be felt in base defence missions.
[HACK WHACK SMACK EDIT]
I'm not in agreement with you on the maximum size of the dropship at 12, BT. Not that it matters much, but my opinion below.
In the original UFO, UGV's and equivalent were weakened so it didn't become a tank warfare game (Health = 90, but great armor). They took up 4 spaces, too, so you put yourself at a significant tactical disadvantage in numerical superiority. If I remember correctly, you could get about 26 team members into the final dropship. This allowed you 4 to 5 UGVs if you chose with a decent number of team members. Heck, the original Skyranger got 14. (Handy link:
http://www.xcomufo.com/x1ufopaedia/index.html)
If you keep ship sizes down at the 8-12 level, you will have made UGV's a completely worthless research endeavour unless they are godsends of firepower and armor on the battlefield, in which case, why care about the men? The only reason to research them would be for the points, as losing 33% or more of your team for *1* of these things seems overkill.
One of my biggest complaints about the old game was that you couldn't hook UGV's to the wings instead of missiles or whatnot, and drop them when you landed. This might be a good way to counteract the smaller dropships, and give UGV's an additional 'bonus', without having to turn them into rolling slaughter machines.
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Now, if you really wanted to mess with people, Winter, as to controlling these things, hand out single handed control units. This would give you your limitations without really hurting the idea. Each human with a controller could control *1* non-AI UGV. Two guys and an arsenal of UGVs at a base could eventually eliminate the entire alien squad. Maybe.
It'd really bite on missions, though. Instead of losing four spots for the UGV, now you're losing 5 (the guy stuck controlling it until it dies). Joy. Maybe that isn't such a great idea, after all...