Because that's where all the important files on your phone are kept and if you screw with things in there not knowing what your doing you can endup with a nice and expensive paper weight. Also, you need size for some of the system files to expand if needed.
Sent from my Liberated D2G
No, actually, thats incorrect in a sense. You will not do anything to the system
as long as you are not editing around inside any of the protected files themselves. This is basically the same as a PC...just think of the internal storage of your phone as the "C:" drive of your PC. The entire Drive is open and accesible to write to, and there will be no issues as long as you dont monkey with the core files...same thing here on the phone. The Droid Incredible allows access to the internal storage...and has no issues in doing so.
Secondly, there is no system file that is going to expand so far as to fill up an 8gb of storage space. Even after formatting and appying the OS, there is still roughly 6-6.65gb of usable space...add in around 50-75 apps and you still have over 4gb of space available...plenty of space for additional music or pictures...
Common sense goes a long way, unfortunately, Moto didnt apply it here when not building in an avenue to use the internal storage...asinine IMO...