Another benefit is most of the nice ones will smooth out and clean the electricity for you, along with giving you 5 minutes of extra uptime. The uptime is also nice if you have to rearrange wiring for some reason. There are benefits, but it's not going to keep you going when the electricity is "truly out".
I don't know if this issue exists on KnC miners, but if you don't shut down properly on say.. BitFury, it could leave the file system in an unstable state. When you restart, it will attempt to fsck and repair the file system.... which usually just screws up the entire file system and you need to reimage the OS from scratch.
So yes, having a UPS to prevent unexpected shutdowns and file system corruption is important to me.