`alias rm="rm -i"` considered harmful?

You're right.

It's bad because you get used to it. If you're on a system that doesn't have it, and you rm, it immediately starts deleting and you're wondering what's going on.

Many users are used to SSH'ing into different systems; so using lots of different systems, sometimes without personalized user accounts (including aliases) set up, is rather common.

Instead, use e.g. alias rmi='rm -i' and learn to use that one. If that isn't set up on a different system, you didn't accidentally delete files and can always fall back to typing the full command.

Like @Daniel said, it's not harmful in and of itself, other than training you to expect it to be there. In fact, it's the default set-up on CentOS (and by extension RHEL, I presume - been too long since I've used one) machines, and it's a massive pain in the tuchus. For the rest of my time at that gig, I typed /bin/rm in order to avoid the "Linux for people who shouldn't have root access" setup.

I think the big danger is that people might rely on something like this to filter a glob. Imagine you want to delete some images from a directory, but not all of them:

rm -i pics/*.jpg

You could use that to filter the glob manually, which would be completely reasonable. But, if you had aliased it and were using rm and happened to land in a shell without that alias and try it... you just deleted all your pictures, oops!

Personally I also find this alias to be harmful to my blood pressure ;). But that's just me.