Yes, the IFV concept has turned out to much less than what was intended. But as long as IFV's do not carry the same or nearly the same protection as MBTs, the two cannot truly operate under all the same conditions, as the vulnerability of the IFV's force the infantry to dismount well before the objective. IF, and that's a big IF, IFV's carried the same or nearly the same protection as MBTs, then the infantry May be able to dismount on or at least a good deal closer to the objective and spend that much less time under direct observation of the enemy. Mind you IFVs like this would be very heavy, very expensive, and a lot of mechanized infantryman would have to get in light infantry shape pretty fast to deal with those situations where armour, especially heavy armour, just can't go.
The other difference between IFVs and wheeled APCs is troop comfort. Heavier, tracked IFV's don't give a very smooth ride, the troops are bounced around a lot. Wheeled APCs give an even rougher ride. I remember riding in the back of a 8-wheeled Bison APC (early Canadian version of LAV-25 with troops seated facing each other) and we were being thrown around inside, and this was just on a gravel road. The earlier Grizzly (LAV-1) was 6-wheeled (although the troops were seated back-to-back IFV style, so weapons could be used from inside the vehicle) was even worse, especially since there was little leg and foot room.