Just think of doing a shrug, with DL's (most any type), as a Super Set for the traps as well as the total back area, as they function together as a unit. Talking BB'ing reps here (8-12) but the same could be said for heavier and more serious sets of 3's for example. If you feel doing individual traps works for you, and your happy with that then it's you choice. Lots of guy's do, with good results. Shrugging at the top position is just another option some might consider. No one is suggestion that anyone should. But that full contraction of the traps, at the full standing position of the DL can make a lot of difference for a lot of guy's. Just like it would for the biceps or any other muscle group.
DL's do stretch the traps quite well, but the ultimate conclusion to this compound movement would be trap shrugs. As with ham's, stretching is good but the actual contraction of those muscles should also be included in ham work. This can work well for pure BB'ing. Want to work the hams for the full potential of progressive size/strength. This can also apply to the calves. To build muscle mass on the biceps you will require more than just stretching them (the DL can be a good stretching movement by the way) you would spend some serious time of all froms of curs. Good Luck.