I just don't like how they fit me. I've tried getting used to mine, but always end up fidgeting with them too much to bother. And I DO find them bulky in my jersey, considering that they take up that much room and only cover my arms. A shell jacket, or even better a shell jacket with zip-off sleeves that becomes a vest, is much more versatile.
Having said that, a pair of mine saved me a lot of discomfort one Fall day. It was 75 F when I left the office for a group ride, and a cold front came through. It was below 50 F, with a steady headwind on the last hour of that ride. I wore them a few more times after that, but got tired of them. Too much bother for how much I get out of them.