MMR for each mode should not be totally separate

I'm an unranked noob, but I played an ARURF against a Plat 3 yesterday. Guess what? He carried super hard and won. Out of his last 20 matches, he's 8-2 in ARURF. He carried all those 8 super hard, while still maintaining >1 K/D in the 2 he lost.

Out of those same 20 matches, he's 1-6 in ranked, where he's probably playing against people close to his skill level.

I'm not saying MMR should be totally unified, but I do think that much of the skills and knowledge gained in one game mode will carry over to others, such that it would make sense for much of the MMR gained in one mode to carry over to the others.