Let's face it, crypt things really don't do much. They have a seriously cool name, but their thing is that they look like liches.

When the party predictably starts hurling fireballs and swinging +3 Holy Avengers, BAMF!, they teleport the characters elsewhere. It's not fatal - it never is - it's just an annoyance. If the party slows down and starts asking things like, "hey! Where'd you put our cleric?" the crypt thing tells 'em that the alive and well characters have been disintegrated. Sort of a one trick pony and only an okay trick.

The first edition Fiend Folio, though, sported an incredibly evocative illustration by the great Russ Nicholson. Instead of a kinda-interesting sorta-skeleton, he showed you this undead thing which could pass the long years, decades, centuries, in quiet reading and contemplation, until the day careless tomb robbers came to disturb its reverie. Cooooool!

Apparently I'm not the only person who was hooked by the pretty picture because, despite their kinda eh power, crypt things have shown up in every edition of AD&D/D&D since.