Maybe with an attractive +buy, you could get multiple Ratcatchers later. You're slightly better off trashing a Rats than you are trashing any other junk card directly, but you don't want to have more than 3-4 Rats in your deck at any time. Maybe you open Rats-Woodcutter and hit $4 with Woodcutter really early. The trouble, I think, is that the "+ card" benefit isn't strong enough to compensate if you can't hit the ground running with Ratcatchers around the time you get your 2nd or 3rd Rats. It would also be fine on a board with junk attacks - you could Rats the junk if your Catcher isn't on the mat. Overall, it's a balancing act, and you need to keep the Rats in check or else you've missed the point. The relative difference is probably small in favor of the combo over just Ratcatcher, if there is any difference at all. And I doubt that it would work without +buy.
Of course I'd much rather have Upgrade, Junk Dealer, Salvager, Transmogrify, or Remodel. Even Develop would be better. I would somewhat loathe Expand as a rat trasher, though, unless King's Court is on the board, and I never seem to have great luck trying to Forge the bastards, either - I think colliding three cards is too hard, even if you've trashed most of your starter deck. You don't really want to keep your Estates, either, so trashing a single Rats along with a different card will hit all kinds of empty or unattractive price points.
