Not only prototyping. If the final product will have a moderate amount of users generating a moderate amount of data, there is really no drawbacks with mongoDB. The real cost is development time.
That's why I suggest it's good for prototyping, maybe it will work for some projects long-term, but I don't think it's the norm as mongoDb would suggest.
However, most prototypes don't end up as successes, so that's where I figure it's worth the switching cost once a go-nogo decision is made.
It's a completely fallacious argument. Using MongoDB doesn't save you time, it just shifts the effort towards a later point, where it will require several times as much effort to fix shit, which may not even succeed and you may end up with corrupted data.
It's really not worth it.