Right, the theory with our mastery and CoP was that they would be able to tune our single target up or down by adjusting either Mastery or CoP, knowing it would be the lead single target choice (which it is). The issue is that since our mastery isn't significantly better than our other stats for CoP right now, having a full mastery set doesn't significantly benefit CoP DPS more than a set with balanced stats, or even a relatively stacked alternate stat - like crit.
If they wanted to, they could buff our mastery slightly, which would make a mastery CoP set more relevant, and of course slightly increase our CoP + Mastery Set single target DPS. I don't think they are that into fine tuning us though - it's enough for them that it works in theory