The main reason for not having a TD is that u can take +9% aim in tank tree not sacrificing some vital talents in AP tree. That ends having 1830 aim in BM/WH gear for me, and I watch shiny 5k crits from RS on PVE-geared targets. All other skills get significant damage boost too.
Also, TD is a harder hitting Explosive Dart, with 1/3 of the range. Many people will prefer the range advantage, since Powertechs lack in that department.