Can you imagine if Brady actually had a decent receiving core around him? He'd be unstoppable.
Welker was the best receiver he ever had, right? (sure, Moss but he was only good for the one season with the Pats) And Welker was nothing special with the Dolphins. But, the Pats system did make Cassel look good for a season so far torn...
I guess you could say that but Moss has records from that season. It was one of the best seasons in NFL WR history...
I think the Pats realized that if you have a good QB he can make the WRs better. They don't spend a whole lot on them (even Moss, IIRC) and it generally works. Sure, let Welker go, plenty of good receivers want to play with Brady.