Melan
Explorer
If we followed this argument to its logical conclusions, we wouldn't need more than a half dozen monsters - Hitter Monster, Caster Monster, Sneaky Monster, Support Monster, Resistant Monster and Mastermind Monster. Of course, this is absurd. But there is a point to designing individual monsters instead of deriving them from basic types, since the available tools can't do everything. Templates, class levels, feats and skills only go so far; they can give you a variation of a standard monster and they prevent, for example, the proliferation of goblinoids which had happened in AD&D, but they can't recreate beholders or rust monsters. They could probably recreate ogre mages (or whatever), but after a while, the extent of changes is so much that the ogre mage is better served by being a separate monster. It is more effective that way.