No one is ruling it out as being the default in the future. However 1) it was a pretty major code change and 2) this is the first release to make it publicly available.
Do you really want that enabled by default right off the bat?
I thought the whole idea behind MongoDB is to deploy at scale. In other words, deploy over 100s of boxes and don't worry about RAID, or single sever durability because at that scale any single server is liable to fail so you'll need to ensure platform-wide durability via replication.
MongoDBs killer feature was/is sharding. If your deployment isn't going to require sharding from the get go, then I'm not sure why you'd be attracted to it instead of any of the other more mature alternatives.
Adding single server durability just gives MongoDB more possible use cases (e.g. small site, single server, low overhead enviroment, etc.).
Currently (mongodb 1.6.x) sharding is a killer feature that might kill yourself. It's just a marketing propaganda.
I once tried switch to sharding to avoid high write lock ratio(MongoDB use DB level lock currently, only one write operation allowed at the same time). After sharding, MongoDB even don't know how to count my colleciton. db.mycollection.count() return values at random. mongorestore also failed in sharding setup, there's no error message when I was restoring millions of documents, but after it reported successfully restored, I checked the DB, no documents there.
"This (mongodb's write plolicy) is kind of like using UDP for data that you care about getting somewhere, it’s theoretically faster but the importance of making sure your data gets somewhere and is accessible is almost always more important."
"It's okay to accept trade-offs with whatever database you choose to your own liking. However, in my opinion, the potential of losing all your data when you use kill -9 to stop it should not be one of them, nor should accepting that you always need a slave to achieve any level of durability. The problem is less with the fact that it's MongoDB's current way of doing persistence, it's with people implying that it's a seemingly good choice. I don't accept it as such."
development agility is a goal of the project too, in addition to scale-out. i've seen a lot of happy users with a single server (or two) and that's all they need.
Do you really want that enabled by default right off the bat?