I think what we're forgetting is that we've never seen him without a shell.
The reoccurring theme of the character is that he HAS a shell.
If he were able to be removed, it would be represented SOMEWHERE. Super mario rpg adds equipment to the base character, that is attachments to the existing character.
It's FAR from unprecedented for intelligent species to add additional defenses onto their natural defenses in both fiction and real life. That includes layers of it.
Skin is a natural defense, we add all types of defenses from basic clothes to full suits of platemail. Bowser is an intelligent species, why wouldn't he follow suit?
Bowser is an being with a shell that can add additional armor and has done so. Because of the sheer size and therefore weight, his locomotion is one 4 legs. The limited range of motion because of the shell and the ackward angle.
Therefore, bowser is regulated to biting and swipes.
Add pretty much all his match-ups to the reconsider list.
No it does not have to be represented, because it
already has.
The Koopa Troopas are the biggest examples of this. Bowser has been theorized to be an evolved Koopa Troopa, which could account for his title of "Great Demon King Koopa" in Japan. If it rings true that he is of Koopa Troopa decent, then shell removal being possible can be a given, because he would be an evolved Koopa Troopa.
The reason for this is that the removal of their shells makes them sacrifice defensive ability for speed, possibly giving them the needed escape velocity to fight another day. Then if need be, find (or in Bower's case, make) another shell. then come back to fight. Not that Bowser would be running away since its is not possible in this thread.
If he is capable of removing his shell like his Koopa brethren, then the only reason he has never taken his shell off is because he probably.
A.) Values it, because it makes him look tough and bigger.
B.) Has never thought to take it off, or never has been posed the question whether or not he can take it off.
C.)He doesn't know he can remove it.
Super Mario RPG however rules out B and C, which leaves the possibility that he values his shell a great deal and doesn't want to leave it. It makes him feel comfortable and safe. Its like a safety blanket to him, he would feel naked without it.