Your vote is definitely counted and I agree, that it's a very powerful feature. I think it's also one of the features that are very original to Mp3tag and one of the parts where it inspired other programs.
However, it's also one of the features where new user struggle the most in terms of installation, updating, and getting the most out of it. It requires ongoing support and constant work to keep the sources alive.
And you can't imagine how many "bug reports" I'm receiving for broken web sources and how many hours went into fixing some of them. I've tried to get all the parts right in Mp3tag for Mac and hastily porting the Web Sources Framework didn't feel like fitting into that vision.