Yes breath attacks can be used once per battle but it can do alot to tip fights into your favor.
I have had games where a single breath attack in the shooting phase demolished the majority of an elite unit *it was a hydra on high elves but still*. Also as you can march and use a breath attack you can put yourself into the sides of those large hordes and get alot of hits on normal infantry.
To me the Firebelly brings some clear expectations, he has a breath weapon, fire attacks, and will pretty much 90% of the time have Fireball as his spell. Of course once the game is large enough I would want a slaughtermaster but that is because he can get enough of the Lore of the Great Maw to likely get the spells I want. A single butcher just to me does not cut it by himself *if he had a way to get a third spell I would consider it*. Also a Firebelly only has to be Lv1 for the use I invision, and only goes to Lv2 if I have two of them and really really need Flaming Sword in my list.