Do you honestly remember your first encounter with a rust monster? To me, a memorable encounter isn't one that is just "poof" oh ha ha, naked fighter.
Orcs, despite appearing in dozens, if not hundreds of published adventures, can still be entirely unique and interesting. Yet, an orc is a pretty boring creature on its own. No special abilities to speak of, just a strong human with a bad haircut. Same with goblins really. Yet, "Bree Yark" is still remembered by long time gamers today.
That's what a monster should be. Granted, it's unrealistic to think that every monster in the MM will acheive this, but, the goal of the MM should be to show creatures that can be used in the widest possible ways. If the Rust Monster appeared in a supplement, I wouldn't worry about it at all. Wouldn't bother me in the least. But, to have it in core means that other creatures, ones that are a heck of a lot more versatile, aren't.