I've never cared for summons, cause in the past they were nothing more than over-bloated Black Mages and in recent years they are either generally not very effective for battle as your main party is usually way more competent for battle. I'm still waiting for them to get the FFX style of summoning to work out and personally I feel its going to need to involve replacing the summoner as opposed to leaving them.

Not to get off topic though, I usually don't care. To me the way it works in gameplay should be based on what is best for the story. Getting job classes from crystals? I want a job class summoner. Summoner is integral to story? Have the clkass restricted to characters. Summons are magic artifacts that he writer says allows the cast to gain super powers? Then lets use something like Magicite, Materia, Junctions, whatever else we can find. You get the picture.