I think there aren't many options what flat shoes to wear when it's really hot to begin with and since most other styles are pretty darn awful - flip flops, crocs, Brikenstocks - there isn't much left anyway. I think gladiator sandals are very cute and they can look stylish with skirts, shorts and skinnies so to me they're the perfect choice for summer.
Of course you can also wear ballet flats, Converse sneakers and such in summer, but I end up swimming in them pretty soon, so I definitely prefer sandals.
I actually had two pairs of gladiators on my wishlist for next summer - a pair by Emilio Pucci and one by Belle Sigerson - and I thought I'd wait for them to pop up/go on sale on theoutnet, thinking since gladiators were said to be out of fashion I didn't need to hurry up with buying them, but once they were in/went on sale they both literally sold out before I could even add them to my shopping bag!