The DSC or ASC+T is not critical. On dry and wet roads with good tires, resonable speeds and a prudent driver (one would expect those that like BMW's might know something about driving) you will not have trouble with a rear-wheel drive car.
I drive with my ASC+T off a lot of the time because I like to control the car's line through a turn with a little bit of oversteer. This is even at very high speeds.
Where the ASC+T/DSC comes in handy is for first time rear-wheel drive drivers, wives of BMW owners (well, certainly not all of these), and in very slippery winter conditions.
That said, DSC is a great safety feature and will probably bail you out at least once during your ownership experience. If you want to get a really good idea of what these features do for you, take the BMW drivers training. They do a lot of manouvers with and without these features turned on.