(This following post is meant to be read in a jokey fashion, no offence intended!)
Ah yes, the universal Law of Conservation of Magic, first discovered by... wait a minute... there isn't a Wikipedia article on this - IT DOESN'T ACTUALLY EXIST!!!
The magic just goes. The rust monster, fine exemplar of the laws of entropy, just ruins it for everyone. That being the point of a rust monster. That you should fear it, not so much for its terrible ferocity, but rather for the fact that it can munch your bling.
I mean, based on what I'm hearing here (and I'm not sure this is correct, please let me know if not), is a rust monster not a million times more useful than a well stocked magical bazaar in 4E? Tired of that tiara of Disney Princess +3? Sell it for a crappy 2,000 gold, or feed it to your captured rust monster for residiuuuuuum to make a 10,000 gp He-Man Power Sword +2!
A rust monster. No home is complete without one.