Rats can also work out well just to trash curses, use a Rats 3 times on curses and it is now a cheap Duchy (with optional trash a card to draw a card, say if you whiff on your engine late game and can spare a copper to fish for a village). Sure most other curse trashers are generally better, but if you have only money trashing (e.g. Spice Merchant) or no other trashing but will see the Rats often enough to clear out your curses, Rats is a good buy.
For instance say I lose the curse split 4:6 but we are both using Xroads, Village, Hunting Grounds, and Witch to draw deck. The game is getting to the point we need to buy Duchies. You buy the duchy, I buy a rats. Turn 0 I lose 3 points. Turn 1, I trash a curse, now I've just lost 2 VP relative to you and my deck was 1 card more space efficient. Turn 2, I trash 2 curses and that means I'm now even with you (+3VP for duchy vs - [-3]VP for curses -> Rats). Turn 3 I now trash 3 curses and now I'm up 3 VP net on you. All the while I have been having a slightly more robust engine. Rats is unusual as a late game "get rid of curses" option in that you can burn through an exponentially increasing number of curses (1, 2, 4, 8, 16, 32), not terribly useful often, but it does increase the span of time where you can easily pair them up with curses vs when you just want the Duchy points now.
Moral of the story is any trasher can be good when it is the only thing to ditch curses. However you need to be able to play it enough to buy it instead of estate or duchy and you have to consider the three pile setup (e.g. I may get more VP using Rats on Curses, but just piling estates ends the game faster). Even though Rats leaves you with ultimately dead cards they still don't cost you VP and for a while they aren't wholly dead (you have some non-zero chance of drawing them with a curse to trash).
In like fashion, some on-trash effects can be strong enough to merit using even Rats as the only trasher. Market square is the obvious example. If you have enough overdraw you swap say an Estate for 2 golds & a Rats. Do this three times and you have on net +1 dead card and +6 golds. While not close to overpowered, it is something to consider if draw is cheap and you find yourself with 2 or more Market square for whatever reason (e.g. 1 for a +buy, 2 because your engine currently does better with another Mrksqr than silver at some point mid-game). You might try Squire (e.g. Rats trashing Squires into Rabble is a net +2 cards after the initial Rats), but its utility is limited. On the other hand even Rats can be useful to pinata open Hunting grounds late game when you can gain actions easily (e.g. Haggler), but lack +buy & lack other action trashing (relatively rare, but fun when you can surprise people with a late game sprint of a Province & Duchy a turn).