I use the summons in this game to save me from certain death.

My favorite is probably Odin because he saves me from dying the most.

Aside of FFX and FFXIII, summons aren't particularly useful to me, though the first few times I played FFVII and FFVIII I would summon constantly...now I'm not sure why I did that. But X's were lots of fun to play around with and could really help you win some of the tougher battles, and XIII's are great if you need to replenish your party and have no easy means to do it, not to mention basically all the hits you get in with them are free.

I agree with the sentiment about the driving/gestalt mode aspect being dumb though. It makes no sense that I can find, and it just looks incredibly silly to see Odin transform into a horse and Bahamut into a hoverboard. But nothing looks quite as ridiculous as when you're fighting enemies by doing wheelies with Snow's Shiva cycle and spinning your tires to make sparks with Sazh's flaming racecar.