He specifically said that he does the strikes to get marks. He doesn't care about xp or engrams. He wants the marks and rep that you get from completing the strikes. The faster you're through the strike, the faster you accumulate rep and marks. After level 25, the end-strike gear is useless, and so is anything that is blue. Hell, even some of the purple shit is still useless. Why waste 20 extra minutes to get gear that you don't want, when you could easily just go through the strike as quickly as possible to get the marks and rep that you actually want?