wands of acid rain

are they good for anything?

melting walls mostly. I usually zap people like Sherry with them too, not sure if it does any good

They're handy to have one or two sitting around in your box for those rare cases you might need them. I can't recall being saved by an acid rain yet, but I like being prepared for any situation. The damage they do is quite minor and often just a wasted charge unless you can keep your targets in the rain somehow.

I think their benefit would be more rusting/breaking enemy weapons and armor than actually injuring them

For maximum efficiency, they should be used with beartraps or golems. Preferably something that doesn't rust.

They're also really good for getting into that one room with the chest protected by meteoric steel walls.
Other than that, chucking a couple on top of a live mine makes it just that little bit more dangerous for whatever ambles into it.