You need to look at cost as well. If you are looking at a large layout of variable speed machines, especially large ones, VFD prices start to add up.
Comparing the cost of a 300-600HP drive to the maint costs of a DC machine gives you a very long time to get any ROI.
Now, if you only have one or two applications for this, the cost of setting up a DC power system may be the offsetting factor.
Just a thought.