It may be useful to allow bulk operations to span collections. I have found that bulk operations tend to be significantly more performant (than multiple discreet operations). In my workflow I have come across several scenarios which would benefit from this feature.
For example, upon completion of a certain program operation, I need to update one table and insert a summary document in another. These two operations are always tied together but it doesn't make sense for them to be within the same collection.
By extension, being able to batch multiple operations across multiple databases would also be useful in my workload. In my case I have 1 insertion thread per collection (and can have many hundreds of threads per collections). A cross-database batch operation may allow me to use a small thread pool to perform the same operation in a more efficient manner.