Note: This is actually a pretty outdated post now. YouTube has updated their video processing and audio support tramendously. Not only do they support higher quality audio, but they also support 5.1 surround sound! I'll keep the original post here for archival's sake. Do I support downloading from YouTube? No. But I'm not going to hide the fact they've improved in the past three years.

tl;dr - It's a terrible idea.

A frequent question I see on YouTube music videos is "How can I download this song?". A common answer to that question is to use a "YouTube to MP3" converter which will take the video and extract its audio stream (AAC), convert it to MP3, and then let you download it. Yes, you get the song, but let's be honest... The quality of that file is pretty shitty.