And it was just
announced that development on Perian will be ending.
OnyX is one of those utilities that can do a lot, some of it cosmetic, some of it trouble-shooting. It is worth having. It is safe in and of itself, although with it the user could get into things they don't know about.
Keep it around, if for no other reason than to empty the Trash those few times an item refuses to be disposed of.