The difference is that the engineers and techs don't have any long range offensive abilities. Sure, you buy a vehicle, you go engi/tech to repair. Of course you do, that's just sensible. But what are the enemy going to do? They aren't going to counter you with engis and techs, trying to run at you with their C4 drawn. No, they're going to go with their rocket guys or raveshaws or mendozas. They're going to go with snipers to pick off the engineers hiding in back, repairing the vehicles.
And sure, techs and engis are good against buildings but they have to get their first and chances are they're going to run into some sort of resistance. Even with the Tech weapons, they aren't nearly as good as the dedicated anti infantry classes. Even a good shotty/rifleman can be a hassle for a tib equipped tech.
Like most class based games, each class has a role, a place and a time to use and although the technician is versatile, it's not always the best choice for you or your team to go with.
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That being said, I do find that repair spam on buildings is a bit more effective than it was in renegade. I've often found that we've been able to outrepair damage from numerous mammoths and MRLS with only 3 or 4 engineers/technicians. It has been many years since I played renegade but I don't remember it being that easy to keep a building functional. If it really is a problem, the easiest solution would be to simply reduce the rate at which an individual engi/tech repairs a building.