There are fairly straight-forward ways to control memory usage in MongoDB, especially if this is a concern for you. All you need is a bit of OS know-how and it works just fine.
We run both large (multi-shard clusters) and small memory (500MB - 2GB) use instances of MongoDB and have no problems.
It would be good to have developers acknowledge that, perhaps, they may not have all the information instead of declaring that something can't be done.