Like the previous posters, I think it all depends on the level of education shared by horse and rider.
For instance, greenies+curbs=no. Beginners+curbs=no. Horses that know how to neckrein=yes. Riders who are aware of their hands+curbs=yes.
Case in point: just the other day there was a thread started by someone who was talking about their curb and how they loved it so much because it made their greenie "collect up like an english horse". Um, yeah, NO. Collection doesn't come from what's in the horse's mouth, it comes from the education of the horse.
Curbs=extra finesse but there's a whole lot of horse/rider education that should come before a curb even touching the horse's mouth.
I love riding my mare in her curb. She barely needs a cue to respond, she (I don't want to say "collects" because I'm sure she's not) "contains" her energy, and is all around a very relaxing ride in her curb.
However, she'll do all those things in a riding halter - no bit - with just a little bit more effort on my part.
I see her curb bit as an award that says "You put enough training on this mare that she's now like a Ferrari instead of the power steering-less Ford she used to be, congratulations!!"
I really feel like you should be able to do everything in a snaffle before a curb enters the picture.
Where I live, I see tons of horses in curbs because "they need brakes" or any number of more ridiculous answers. I feel a little bit of pity for them because the way I see it, they've stolen their "award" (the curb) from the ringmaster (their horse) before the ringmaster gifted them the award and congratulated them. I think they're missing out on a beautiful thing because they chose to rush. I feel bad for them because they don't know the feeling of accomplishment that you get when you successfully train a horse up from snaffle to the curb.
Just my thoughts. :)