One of the arguments for DMUs is greater efficiency and flexibility, as well as greater acceleration. Locomotives tend to cost the same to run whether they haul 1 car or 6. While a DMU, with smaller powerplants since they only have to worry about moving themselves, theoretically has a lower operating cost per unit. You can run 1 DMU or a bunch depending on traffic needs and get some economy of scale. Also, many argue that they accelerate and keep schedules better than a locomotive, which is also hauling its own weight around in addition to the cars.

I don't have any figures to back this up, these are just the reasons I have heard advocates express.