If the caster needs to be up on a raised platform surrounded by a chasm, he'll have Fly cast so that he won't fall into the chasm, and so he can leave the platform if he needs to. Otherwise, he's not smart enough to live. Really, if I were a BBEG, I'd run through as many "okay, when the inevitable rag-tag band of adventurers gets here, how will they try to defeat me," scenarios as possible and figure out how to defeat each one. Fly is a good way of getting out of bad situations, as is a Silenced Dimension Door, if you're high enough level or have a (conveniently inexpensive) rod of metamagic.
If the BBEG paints himself into a corner, he's an idiot. Of course, you could always bring back the Vocalize spell from 2nd ed.