Sphere Grid/Skill trees/etc are an illusion of choice. Yes, you can spec however you want but there will always be an accepted "best" once the min/maxers work out whatever that is and if you chose something else have fun being excluded/ostracized for it. The only way this would work is if end game content was retooled around it, where say a BLM who decided to be more of a Fire mage would be the obvious choice for the ice raid boss, but this then fragments the available pool of players you can find to clear the content with.