Oh the edge cases are legion:
1. Trashing curses for the VP gain (maybe up to +10 VP if your opponent pulled Amb out of the Bm deck). On a similar note going after Silver & Gold with Bandit fort out or singletons with Wolf den is quite viable (e.g. a late game Rats to burn a prize like Followers can be a slightly cheaper Duchy.
2. Converting card types - Pool, Cityq, Lib, Grobber, etc. can often work better with dead Rats than dead Coppers/Estates.
3. Gaining a unique(ish) named card. A dead Rats can absolutely be worth it (e.g. other getting 8 VP Fairgrounds with Potion and Bane) other cards that care about diversity (like Menage) can benefit from converting a few coppers into something less likely to collide.
4. Gaining Knight or other opponent trashing defense. E.g Alchemist x5 -> Rats (trash C, gain Rats) -> Courtyard (draw Rats/topdeck Rats).
5. Improving Crace odds (e.g. Kc/Crace/Rats; vastly better to have something other than copper to show)
6. Milking landmarks. Defiled shrine, Tomb, and a few others make it viable to grab Rats without a way to trash them.
In general, trashing Rats does not net you any draw over a straight trasher, Chap for instance will draw a card, but you could use Rats ... or just trash things directly with the Chap.
Also it takes two Rats to pile out the Rats pile. The +1 card comes before the gain. You can also skip the Fortress if your hand/deck are empty (e.g. you duration/mat away your green or have it in play).