I do have a quite a few apps that I never use. The reason is almost always the same: I like the app, but it's missing something, or part of it doesn't work correctly, etc. I keep it so that when it gets updated I'll know, and I can see if the app has been fixed, or has added some functionality that I like.
For example, I use wonder reader and rss central for my rss feeds. However, I love the way flux looks, especially if you replace the background with a plain black screen. If it loaded feeds in the background like the previously mentioned apps did, then I would probably switch to it. I keep it installed so that, if it gets an update that adds periodic background loading of feeds, I'll know.