Blake Griffin won as a "red-shirt" rookie, didn't he?
I don't think it's unfair to include players who have been injured and are thus included in the next year's rookie category. If anything, while true rookies were playing in college, these others were rehabbing injuries and missing playing time.
Simmons is a great player. To argue he is undeserving because he's not the best player on the team is insane. As a Jazz fan, one could argue Donovan is helped by having other shooters and a tremendous screen setter in Gobert. Both are sometimes very inconsistent; that's what being a rookie is all about. Either choice for ROY will be great. And either choice will leave a very deserving player out in the cold.
In any other year where there is 1 clear winner, i.e., Blake, then that's fine. But in a year where there are 2 deserving players, then to me you have to look at other things like 'are you a true rookie?' To me that's ambiguous with Simmons because he got to be around an NBA team for a whole year, seeing how plays are run, watching other NBA players and learning about their tendencies, etc. Also don't you think listening in on coaching sessions wouldn't have helped him improve as a player far more than other players who at the same time were in college? How much better did Simmons got given all the things he learnt last year that Mitchell didn't?
If Simmons is considerably better then yeah give it to him. But when they are this close - you've got to give the edge to the 'true' rookie, and that's Mitchell.