There's a lot of reasons to avoid poison that have nothing to do with arguments about humane treatment of animals.
Consider: secondary killing. Lots of things will opportunistically feed on dead or dying rats, and get poisoned and get sick or die as a result. If you succeed in killing half your rat population in a 500 yard radius, at the cost of killing 90% of the predators that eat them in a half a mile radius, the rat population will rebound faster and stronger than the predator count will, and you'll end up fighting bigger waves of insurgent rats. Of course, that only applies to areas where there's both rats and rat predators, so if you're in a massively built-up urban area with little or no green space, you can dismiss that one.
Then there's the whole carcass disposal thing to deal with. Most dying rats aren't going to crawl to a convenient place to die, they'll die in the same places they live, which means wall voids, pipes, brush piles, under raised floors, above suspended ceilings... basically everyplace that's hard to look and inconvenient to clean out. And if you don't find and remove the dead rats, the place is going to REEK for a long time. Of course, that's only really a valid argument in structures in which people spend time... so if you wanted to poison the rats in your barn, that'd be a little less relevant.
If the rats are indoors, you're better off killing them where you live (out in the open) than where they live (in the walls). Snap traps are the way to go for that.