One of the interesting things about design and development is that it's very hard to predict what people might find disagreeable.
The 10 minute thing is there to contain what I call the creature's "blast radius." A monster's blast radius is a measure of how much it affects an adventure beyond the encounter it appears in.
For instance, in a campaign I ran the PCs fought a demon that could dominate its victims and give them telepathic suggestions at a distance. That creature had a very large blast radius. While the party drove it off when they first fought it, it had secretly dominated the party's fighter during the encounter. For the rest of the campaign, the party had to cope with the fighter occasionally dumping out his potions instead of drinking them, refusing to attack a monster, and so on.
For the rust monster, I wanted to contain its blast radius to the encounter in which it appeared and a few encounters after it.
The question I have is this: if the 10 minute limit had a satisfactory explanation, would that be OK? Or is it just the idea that the rust monster has a very hard time destroying items?
I originally thought of the rust effect as a sort of curse, a temporary, magical transformation that the rust monster slowly makes real/permanent when it really starts gnawing on something. Note that the rust ability went from Ex to Su in the article, though I didn't call that out.
MarkB's idea is awesome. I really like the idea of giving a rust monster disarm.