I dunno why we're not using them, either. On other sites, I have seen no complications, so, I have no idea why then are off here...
Originally Signatures were turned off because they Ate alot of the sites bandwidth (As did Avatars) I would guess it's possible to Ask Porfiry to turn them back on, however the ultimate question is how would it effect the forums? (Would it cause Database overloads or slowdowns? etc)