A new feature lets you search by lyrics. So you can find songs in your library by entering a part of the lyrics of that song.
Of course you can also search by lyrics in iTunes Store.
Arguments: If you hear a song on the radio, and you only catched a few lyrics of the song, you can search the Internet and maybe find some songnames. Then you have to search for these songsnames in the iTunes Store. Much more comfortable would be a direct search in the iTunes Store.
Also if you are looking for a song in your library, you sometimes don't remember the song-title but some chunks of the lyrics, therefore a integrated lyrics-search would be really helpful.
While doing some investigation on current plugins for iTunes, I found the blog-entry of Brad Lauster, which also had the same idea. He also mentions another feature:
...and think of the fun you could have if you extended this idea to Smart Playlists?
You could create a playlist that only included songs with the lyrics "big" and "butt." This playlist would, of course, include Sir Mix-A-Lot's "Baby Got Back" and Nelly's "Hot in Herre." ...just think of the possibilities!
Why it's not already done: Propably because the music-industry doesn't want to provide the lyrics and because that feature is rather nice-to-have
Questions: What other applications can you think of beside lyrics-search and smart-playlists depending on lyrics, once lyrics would be accessible through iTunes?
I found a AppleScript that searches for lyrics, but unfortunately it seems to search for the lyrics in the mp3-file and not in an online-database as I suggested here. iTunes lyric search uses online-lyrics, but seems to work only on Windows. I haven't tested this one yet.