You mean, say,
Ping? LOL
As I stated, they're purely cosmetic when it comes to steel shafts.
Now, I've never, ever seen
Ping irons with graphite shafts.
I've got some barrel-topped kids clubs, and have worked with the P5G's (ping clone) and could use ferrules there (but didn't).
There must be something to barrelling the top of the hosel that makes ferrules a non-necessity with graphite shafts. What makes ferrules important with graphite shafts for standard irons (and woods, for that matter) is that they help create an epoxy cushion that helps shock absorb at impact. Without ferrules for graphite shafted regular hosel clubs, you'd experience much more shaft breakage (or at least it's what I'm told) as the stress occurs sharply at the top of the hosel with nothing to relieve that stress.
Similarily, I've taken to coning all the hosels that I use graphite shafts for, and use collared ferrules for them as well (to further reduce the stress). The guy behind Billy Bob golf (a crazy ferrule-nazi) states that collared ferrules reduce graphite shaft breakage to less than 1%. It just seems like a good idea to me (rather than have to do a reshaft on my dime and then try to negotiate a warranty replacement).