So what exactly do people that are way overleveled gain from running Alliance Roulette over manually queing for their 5 favs of the Alliance raids?

They obviously don't need the exp. After all they clearly want to take their time with the MSQ, which means any and all end level content is far out of reach due to being locked behind said MSQ.
And they can still do any of the other roulettes if their need for poetics is that great.(which I seriously doubt with how few Times I can remember not being at cap for those).