I don't know; in a deck without any TFB, buying a rats really seems exactly like buying an Estate. Playing it adds 0 value to your deck, and quite often you would rather just hold in it your hand than play it.
I'm not sure. Its a tricky card to evaluate for sure.
So, the first one you buy seems questionable. It's a cantrip, plus you have to trash a card from your hand (hopefully an estate). So the first time you play it, it doesn't do anything meaningful to your hand (estate wasn't helping and if you hadn't bought the rats, you'd have the next card in your hand anyway), and its only impact on your deck is swapping an estate for a Rats, which doesn't
seem useful without TFB cards.
But next, consider the possiblity that you draw a hand that would have been EECCC. BUT, instead you've replaced that first estate with a Rats "for free" (didn't spend a buy on it). So now you get to play that Rats and draw a legitimate extra card to your hand. As a side effect, you have to replace the second estate with a Rats, but the point is the initial Rats purchase made the first hand it appeared in no worse, and now actually made this hand better. And if you're getting hit by curses or ruins, this kind of process can continue, giving you a cycling benefit as long as you have junk cards to turn into rats. Plus, its obviously extra good if you're trashing curses, since it actually nets you points. And at some point, sure, you want to stop playing your rats and just treat them as junk in their own right, but hopefully by that point they've already helped you.
I guess the point is that buying cantrips is a waste of a buy, since it doesn't do anything. But a single rats purchase can replace your junk with essentially limited-use cantrips (as soon as they would force you to trash something good they themselves are just junk), which could have a non-negligible and low-risk deck acceleration effect. I'm not sure if this would be helpful with trashing coppers though, since that seems like it would be hurting your deck, but against cursers or looters... maybe it would be worthwhile. Again, its hard to really visualize how helpful or not-helpful any of this would be in practice, but while we're theory-crafting I think there's a case to be made for properly played Rats having a positive impact on some decks even without TFB cards.
Really looking forward to seeing what simulators have to say...