Y'know I never speak up in these threads, but everyone seems to forget a major reason to keep them small and a little boring right now.

Room for expansion.

That is to say, if the game goes on long enough and the Summoner class is expanded upon in updates to come Square Enix will want room to make the summons cooler and more interesting at higher levels. So keeping it simple here during the early days keeps them from having to play a game of one-ups-manship against themselves everytime they want to upgrade things a bit.

One day you might have the ability to have Titan-Egi 'hulk' out and use Rage of the Earth for high, cool-looking AE damage for example. But if you have a big badass titan by your side now, that won't be so special if you do get it.