I also spammed my GFs for the first two discs on my first time through the game, and for a while on Disc 3 as well. Quezacotl, Ifrit and Shiva just were fast to cast, they did a lot of damage and I was able to Boost them too, so I kept using them a lot. On the other hand, magic and physical attacks were a lot weaker. Then I noticed how much better the Meltdown + Aura + Renzokuken (or other limit breaks) combo is, so I started to use that one. On my second game, I pretty much used a mix of everything since the junctioning made my characters overpowered in about every way.