Should cars only be offered with their real-world transmission options?

So for example, if a given model only comes in AT, or in MT, then you'd only be able use that respective transmission when using it. And if a car offers both, then you'd be locked into that transmission after buying it, as opposed to switching back and forth in a menu. I think it'd be a good incentive to get multiple versions of a car - perhaps one to get initially to practice with, with AT, and when you get comfortable, you can get one with MT.