Aside potentially hindering your party? Nothing. You lose far too many stats try to compensate it with extra crossclass skills, so you can queue class if you want...but you're only hindering yourself and your party in the long run. While Emerald and Topaz carbuncle are essentially Garuda and Titan, they're just weaker versions. You lose out on Regen and such by not being a WHM and so on so forth.