I usually avoid the forums like a plague but I wanted to log in and say thanks for all the work you have put into this!

I find the engi spec quite useful in nightmare and HM if nothing else else then for the added defensive capabilities / damage mitigation. The more sheilds / balistic dampeners / coverprobes / i have added into the mix the better. Its less time that a healer has to worry about me and can focus on keeping the tank or other classes up. As for damage, I have never really had an issue with being able to have that side covered. Go through whatever rotation your situation calls for and move / refresh dampeners. Its nice to be able to pop my defense abilities /take a Bonecrusher swipe / still have a good chunk of health left. To me its more about what you feel like playing. I tried out MM and lethality as well and I just liked engi better.