UWA's are fun! They aren't 'easy' to use because of the extreme perspective, but, they are quite useful for many types of shots! You do have to consider the elements in the shot, where with something like a 50mm it's 'easier.'
Sometimes it's not just for tight spaces either...
Here's a few with people. First is from a same sex wedding, the other 2 are from a very small wedding with the same group of people with 16-35, then a 50 1.4. Last is from a larger wedding.
keep it or sell it? It really depends on what your doing with it. UWA's have a special look to them, but, you do have to know what your doing with them to get the most out of them, and you should have some other lenses for a more natural look. If your just looking to get good shots of the fam on vacations, sell the 16-35 and snag a 24-105