I agree with Dingan. Just thinking "Rats replaces other junk with Rats," it can be easy to ignore the opportunity cost of buying the first Rats. You see a board where you have some good junker that you can play along with Rats, e.g. Junk Dealer, and if you think:
"It will be easy to get rid of rats, and I'd rather have Rats than other junk (because of the +1 card bonus when I trash it with junk dealer)"
you are 100% correct; however, if you conclude from this reasoning that
"therefore, I should buy a Rats,"
you are wrong, or at least, you are maybe wrong, because you have to give up a turn to get the rats in the first place, and there's often something better you can do with that turn (for example, it's not always true that the replacing-junk-with-marginally-better-junk effect of rats in such a deck is even better than just buying a silver).
Actually, even without the opportunity cost of buying the first Rats, Rats' on-trash ability doesn't make it worthwhile. Using Rats as an intermediary trasher makes you trash twice for each junk card you eliminate, and trashing a card from hand reduces handsize. Compare, say, Rats->Junk Dealer to just Junk Dealer:
With just Junk Dealer, each starting card (or Curse, Ruins, etc.) you trash goes as follows:
- Play Junk Dealer from hand (-1 card), draw a card (+1 card), trash junk from hand (-1 card). Net handsize change: -1.
- Total handsize cost for each junk card trashed: -1
With Rats->Junk Dealer, each starting card (or Curse, Ruins, etc.) you trash goes as follows:
- First, play Rats from hand (-1 card), draw a card (+1 card), gain Rats, trash junk from hand (-1 card). Net handsize change: -1.
- Later, play Junk Dealer from hand (-1 card), draw a card (+1 card), trash Rats from hand (-1 card), draw another card (+1 card). Net handsize change: 0.
- Total handsize cost for each junk card trashed: -1 + 0 = -1
Rats' on-trash ability isn't a benefit; it just compensates you for having to trash twice instead of once.
IMO, trashing with Rats just to trash them with something else is only potentially beneficial if the other trasher is a scaling trash-for-benefit, or can trash Rats but not your other junk (e.g. Hermit or Watchtower). There are doubtless edge-cases (e.g. Market Square), but in general, you shouldn't buy Rats for its on-trash ability.