Well, for me, I want to do it :
1. Because it'll be fun to do
2. Because by installing Linux on the PS3, I can maximize my investment in the hardware. It is a full blown computer after all, so why not empower it to act like one?
3. Because even though Sony wants the PS3 to be your media hub, it can't natively play certain media files which may be commonly used eg. certain .avi files. It doesn't appear to have a convenient way to get the right codecs etc. setup on it. If I install Linux, then that issue goes away nicely since you can easily get the tools you need to access various kinds of content.
4. Because I'd be able to stream Internet Radio and watch YouTube videos from my comfy recliner in the TV room now.