My experience is this: MDM will save you from the massive repair bill that rldzhao experienced, but I also agree that DSC off is the only way to truly advance in car control skills.
I have an E46 M3 that is race prepped. I drive that car w/ DSC off exclusively. I have spun off the track, but I have been lucky enough to have spared the car from any damage. I drove it at Watkins Glen w/ the BMW CCA one weekend in the E46 and the very next weekend w/ NASA in the E92 (I didn't have rain tires for the E46 and it was forecasted). It was the first time I tracked the E92 in literally 1 year. It was amazing not just the difference in the two cars but the difference in my skill level from the last time I drove the car. I decided to drive the car in MDM instead of DSC off because I just didn't want to risk the car any more than I had to. I know that I could have gone faster w/ DSC off, but I didn't have to "save" the car on two instances where the back end would have came out. I was able to straighten the wheel and the system straightened the car out. If DSC was off, I may or may not have saved the car.
Keep this in mind: if you want to go from MDM to DSC off, just back it off in corners where there are big guardrails/tirewalls nearby. Turns where there is a lot of runoff work much better just in case things go wrong. I am not advocating unsafe driving because the last thing you want to do is spin across the track and get t-boned by another driver, but you really need to manage your risk. If I had more experience w/ my E92 on track I would drive w/ DSC off and be a little more careful as the MDM started making me lazy.
"The laws of physics cannot be repealed, even with DSC." - BMW Owner's Manual