Rats is such a well designed card.
I remember the first days of Dark Ages, some people thought that card would be the worst card ever. It turns out that Rats are fine, you only need something else to combo rats with, and that something else appear in a reasonnable amount of games.
I think usually, Rats + Another trasher (when well played) is a little bit faster than the other trasher only. Ok, Chapel clearly don't need rats, but for something like steward or amulet, the answer is not that simple. Rats being a cantrip, it pair nicely with other trashers, you don't fear terminal collision.
So, if Upgrade is in the kingdom, I would often open Rats in 4/3. In 5/2, I will maybe skip rats and go for upgrade directly, but not always because upgrading rats is such a good move. If Junk dealer is in the kingdom instead of upgrade, I would more often skip rats because the fact junk dealer is not a remodeler makes a really big difference.
Also, Rats by itself trashes only 1 card, so you can still build your deck while trashing. It's not like other trasher where you always end up with $2 or $3 buys. With rats, you easily hit $5. Of course, at some point you can end up with crappy hands full of rats, but that doesn't happen early game, at least not before the third reshuffle. So, before being stuck with rats, you can prepare the following of the game, getting that other trasher and maybe some villages if that other trasher is terminal.
Also, I love forging two rats into province.