I personally think it's just the in-style at the moment. It's different than what we're used to, barebones, and uses vibrant colors -- the antithesis of recent popular styles. Some seasons blacks and grays and navies are in, other seasons women like wearing greens and oranges and yellows. UX-wise, though, flat has more problems, but people might not care because it looks so fresh.
Agreed, it'll probably settle down in a few seasons to something moderate (i.e., neither the garishness of apple's calendar, nor the usability nightmare of the WP8 app on windows.) I just wish we won't forget the lessons of these experiments, but most likely we'll just be reinventing the whole thing again and again. (Recent example: the horrible 90's emoticons craze is back in messaging apps these days, except they're now higher resolution and are called 'stickers'.)
Facebook, Path, MessageMe, and more call it stickers to differentiate from emoji. Stickers are generally larger and can be used on their own, while emoji are used with other text.
