[yummyfajitas]

At my last job we did that on a regular basis, for the most part without even having downtime.

[raverbashing]

And at my last job no one would risk it because any issues with this would mean loss of revenue, angry customers and people having to revert code at a whim.

(Of course, not only migration issues, but any code issues as well)

[kasey_junk]

MongoDb is not going to solve development/operations issues, which is what you are describing. If anything, it makes those things worse as it is much more cavalier with your data.