Not being able to bail from your vehicle is probably my favorite change.
Where have you read/heard that? I've heard only that certain vehicles will be class specific, so that a tank driver can't fly planes and a pilot can't drive tanks, for example. That most likely does not mean that a tank driver is restricted from leaving the tank. That would be absolutely stupid, both realistically and fun-wise. There are also other vehicles, like cars and horses, that are pretty much pointless to even have in the game if you can't get out/off of them.
I imagine that a tank driver will be able to drive/control and hop between just about anything besides planes (or zeppelins) and a pilot will be able to do the same with anything besides tanks. In other words, I expect a little restriction, but not nearly enough that it's not BF anymore.
Beyond stressing expertise, another reason could be to curb abuse. For example, in other BF games, snipers could fly planes to hard-to-reach locations, only to abandon them and snipe from those positions. If DICE restricts things so that you can't be a sniper
and have the ability to fly, then they greatly reduce that problem.
Maybe you're referring to something else, like pilots not being able to eject in flight (since there were no parachutes) or players not being able to exit vehicles that are
in motion. Those could make sense. Not being able to exit vehicles at all makes no sense, though.