Lots of apps rely simply on a Facebook log in. At this point it's become safe to assume that most people using your site or web app will be using Facebook, so if you're not concerned about users who don't want their account to your site linked to any of their other accounts, go ahead, just use Facebook log in.
However, you should keep in mind that most people seem to prefer not doing that. There was a post on HN a while ago about someone who did a quick and dirty google survey and asked whether people used their facebook log in on websites. Turns out that most people would prefer not to.
So, while many apps do do it, I think it's best practice to provide an alternative. So, while you may be dreading developing it, what's more important is your users, and how they experience your app. If being forced to use Facebook turns them off from your app, do you really want that to be an obstacle?
Edit: here's the link to that blog post: http://www.jamiegrove.com/user-experience/34-5-of-us-internet-population-not-using-facebooktwitter
And here's the link to the HN thread, note the first response - there are some problems with the way the survey was conducted http://news.ycombinator.com/item?id=4495934