Sometimes you do just need to dump crap into the thing to see what it looks like, and Lorem Ipsum is better than banging on the keyboard for a few paragraphs.

I think this is a bit of a false dichotomy anyway -- you can do both, use filler text and viable copy when it's available. But if, say, you're making a forum or have a client who isn't exactly eager to dump copy on you, or they want to write it themselves, or you just want to tweak around on some css, anything is better than nothing.