Honestly, I pretty much gave up on trusting YADB (or CDDB or freeDB) quite a while ago. Even if the online service finds something, I still have to verify it by hand to make sure it's correct. I've never turned off the online lookup stuff, but the only thing that I look up is CD info when I put a CD in. Even then, I find I have to hand enter it all anyway. It amazes me how difficult it is for some people to just type in what they see on the CD. It amazes me even more that even though they can't be bothered to insure accuracy, they still feel the need to upload it to YADB. Whenever I have to correct a CD lookup, I always submit my changes. Whether they get used or not... I have no idea.
I gave up on cover art lookup because it seemed to rarely get it right, and what it did get wasn't a high enough quality to suit me. So I turned to Google Images and most recently, Album Art Downloader.
And doing an online lookup for an individual track or a bunch of tracks? Forget it. I have no idea WHICH fields doing this is going to replace. If the only thing I want is a date, then the last thing I want is MC fetching a bunch of incorrect data from YADB and replacing all the tags that I knew were right before doing the lookup. Sure I can Undo the changes, but that requires me to look at almost every tag to make sure it didn't overwrite something I didn't want it to. Maybe if it gave me the option to select which fields to fetch data for, or gave me someway to review the proposed changes then I'd use it, but as it is now, no way.
With all that said, since I'm so meticulous about my collection, I figure if somebody else can use the data I've collected, go for it. I guess I just wish I knew if this was only happening with audio files or with images, videos, and documents as well.
It would also be nice to know what happens to the data once YADB has it. If I manually change cover art and MC automatically uploads it to YADB, how do other people get to use it? It's a voting system. I get that. But what if MC returns 500x500 art, and I scan in a 1200x1200 copy and upload it. According the tallies, the 500x500 has a bunch of votes for it because everybody's used it. But mine has no votes because it's brand new. If I later do a cover art lookup for that album, which art is MC going to deliver?