I used to hang out at the alt.music.lyrics newsgroup, which handles hundreds of song requests a day. The structure of many newsgroup readers is such that it's often very hard to find the response to a request, so most people ask for an e-mail reply. I'm sure newcomers don't understand our structure here, so that's why they ask us to reply by e-mail. They also get confused by our threads disappearing after no posts have appeared for 24 hours.
We find all this stuff very easy to understand, but Mudcat is so full of information that it's overwhelming to the average newcomer who's just looking for a single song and not intending to spend a lifetime here.
So, I figure it's best to both post and e-mail a response when a requester provides an e-mail address, even if they don't ask for an e-mail reply. If you do send an e-mail, I'd suggest that you note it in your message in the thread.
And yeah, I find that most people do e-mail me a word of thanks if I e-mail them lyrics. I've made some nice friends that way.
-Joe Offer-