I think it would be odd if you can't use Carbuncle as a Summoner. Mostly supposing they do a system like XI where you have to go out and fight the primals before you can summon them or do a summon quest, how would you be able to get things done? Sure you may have Debuff skills, and I'm sure that book can do a decent amount of damage.. probably not, but a summoner is nothing if they're not summoning.