When I bought my 60D, the first lens I purchased was the 10-22mm (similar to the 16-35 on a full frame). I used that as my only lens for nearly a year before buying a 24-105. During that year, I learned a lot of things:
*With an UWA lens, you need something close to your camera to be the 'anchor'. If everything is far away, it all looks flat. This is even more true on a fisheye lens.
*An UWA gives context to a subject. If you're taking a portrait, and you pull out your 70-200, and get a nice head and shoulders portrait, it will look great, but it will also look like every other portrait. By all means, get that shot, and then pull out the UWA and get a shot of the person doing something. In my opinion, the 70-200 captures the subject, the 35-50ish gets the subject and shows some background, and the sweet spot is the 16-24ish, where you're "telling a story". It's hard to tell a story with anything longer than 50mm. I'm not saying it can't be done, just that it's difficult.
*An UWA gives you a greater depth of field even at the wider apertures. Pop it up to f8 or so, and you almost can't miss focus. On my 60D with the 10-22, I really never worried about focus unless I had something within 2 ft. of the lens. That's a nice thing! If you take a shot, and you just slap the focus somewhere in the middle, and you're around f5.6, it's pretty guaranteed to be clear.