I think of it this way, jb. An effective scorer in the post is much more consistently dangerous, and typically has less ability (SPD, BH, PA) to counter a DT, whereas a scoring guard is typically more able to counter, especially when he has good IQ. I like to DT when post players are scorers, and also lack PA, SPD and BH (and IQ hopefully). It's rare to find a scoring guard with similar deficiencies, but if you come across one, a DT just might work.