First, NT supports mount points. In fact, a few of the partitions on my system are accessible only via mount point and not by drive letter.
As for why MSFT keeps using drive letters, it's all about backward compatibility. Apple is not afraid to arrogantly say "f*ck you" to all of its existing users (e.g., the new Mac Pro does not support Firewire 400, leaving all existing iSight users screwed... Apple pulls this kind of stunt all the time with hardware and software). It helps make Apple "cool", but it makes Apple a very poor platform. Microsoft realizes that in order for a platform to be successful, it has to be, first and foremost, stable, so people who invest in the platform can feel safe knowing that years down the line, what they bought for (or developed for) the platform still viable. That's why something so fundamental as this will never change because every MSFT OS has used it, that's why Microsoft bends over backwards to ensure as much compatibility as possible, and that's why Windows dominates.