I've usually used pymongo  for my API docs, and I don't believe I've ever seen this limitation listed there. I also rummaged around the admin area of the mongodb site and don't recall seeing the limitation there.
I'm really glad I haven't deployed mongo now in a production 32-bit system.
Response to EDIT2: Where can data loss be acceptable?
If you are having a relatively speedy message system where messages are removed/outdated on rx. I'm sure there are other specialty needs.
For pymongo there is a "safe" parameter you can apply to operations or the connection as a whole. 10gen made a really stupid default decision here. Instead of calling it safe they should have called it async, and it should have defaulted to off.
So by default Mongo write operations are asynchronous and you have to explicitly ask for error codes later.