Splitable views and tabs are mechanisms that will probably solve all the problems people want MDI used to solve, but in a much better solution.

In fact, having split views on a single document in a single window along with having a separate window for each document will probably be the best solution.

Adding in tabs will be a solution for some people, but personally I don't use them nearly as much as others.

Having both implemented and allowing the users to use as little or as much of each as they desire will probably maximize productivity, and that will be far better than trying to solve those with MDI.