Rust monsters have been around since 2nd Edition, at least. Maybe before? I never played an earlier edition except for Basic D&D when I was in elementary. Rust monsters are a critter that I never got around to using and thankfully encountering. I didnt want to use a rust monster since if it managed to destroy the party's gear, that I would have to incorporate some way for them to get new stuff or bulk/swap out the items that they were going to find.
In other words, it would have ultimately been pointless and just added more work for me to do.
Honestly all the rust monster seemed to do was perform as a game-stopper. "Oh noes, it ate the fighter's magic sword and armor! Now he's useless until he gets another set of magical gear since he is the most magic dependant class ever." Seriously, whats the point? All it did was make it so that one player has spend the next x encounters waving his hands around in front of the wizard as a literal meat-shield. Thats neither fun nor challenging, but simply random, making it no better than a save-or-fuck-off effect, which is part and parcel to a terrible design move.
Now that the rust monster has been officially re-released into the wild, some people are complaining about the new mechanics like they are a bad thing. Frankly, these are also probably people that dont even play 4th Edition, but want to bitch and whine about how Wizards isnt making more books for their legacy edition instead of rightfully leaving to die in the ditch it was cast into years ago.
The new rust monster fulfills its design purpose and also allows for more interesting combat challenges. If it hits you, it starts damaging metal weapons and armor, which is reflected by an attack/AC penalty. Why is this good game design? Because it doesnt utterly obliterate a character's prime method for doing anything for much longer than the planned battle.
Its still dangerous and characters packing metal will want to avoid it because it gradually makes them eaiser to hit/harder for them to hit other things. Players unfamiliar with the creature wont be randomly hosed by it from the start, but will be affected by it, yet be able to react in an intelligent, informed matter...which is excellent game design.
It can still destroy one magic item per encounter. The item in question has to be rusting for this to work, so it requires a bit of setup. This would allow a player to discover that metal doesnt work too well and keep her distance, though a lost item can be recovered at the end of the encounter by harvesting the consumed residuum from its carcass.
I like to think its a matter of not being a "dick DM" by throwing all this random nut-punching bullshit at my players. Not wanting to destroy their gear isnt the same thing as not trying to, "make nine-year olds cry," and it certainly doesnt make the game less challenging. I think its a pretty big leap to get from what amounts to grinding the game to a halt as you arbitrarily punish and cripple about half the party. So you set them back quite a bit of treasure, now you have to make it all back up or reduce the danger of your encounters in the future. Whee!
The important thing is that rust monsters are still bizarre illogical things that matter. They are still feared. Just not for me as a DM. I can comfortably throw them at my players without worrying about having to revamp future encounters and/or treasure parcels to make up for my dick-move.
Of course, I could also just buy my players dinner.