I totally agree with you.That would depend on actually what type of contact that they have, if they are coming up against a mechanized unit then I see your point, but if they are clearing out wood lines or towns or villages then they are sitting ducks inside of their IFV`s, there may be occasions where you have no choice but to dismount to clear out a well entrenched enemy.
BMP`s series vehicles are designed for carrying infantry to the battle, and you are right, with the offensive style tactics that are used by Russian forces or any sizable NATO armored force why dismount if you do not have to, let your artillery keep them pinned in place while you out manuver or by pass them. The name of the game is to eat up as much real estate that you safely can do without out running your supporting units and make your opponent react to your moves, be it offensive or defensive, if he has to react to your moves by throwing in additional assets or reserve units then he has lost the game. Dammit - I wish I could get back in a tank!
I just wanted to show that you just cannot say that the BMP is not designed to carry infantry under fire.
It totally depends on the situation and I just stated some situations were also BMPs are used for transporting infantry during a battle without dismounting.