i think it would be useful and practical, and having gone through the effort to consider the difficulty of implementing such an approach, if you were going to write a sim engine from scratch, its not that difficult. but my guess is to shoe-horn it into the HD engine, the effort would be somewhat higher. i dont know how they do defenders exactly, but one would think they could give us something most of the way there fairly easily. for example, if you could only use this in m2m, and in m2m, the HD sim engine simply thinks, the primary defender for player X (on their team) is player A (on your team), because both are currently playing PG - you'd think you could specify who you want your defenders to primarily defend, and the HD engine could substitute who the primary defender is, without that much trouble. if the whole team is being used to calculate defensive values, it might be tricky deciding how to weight the remaining players, but i honestly dont think it could be that bad. not at the WIS standard of excellence, at least :O
there are some complications that arise to really get it right, but that would be a much longer post, so i wont get into it.