Solution for the folks whose secondary role actually is support...
I'm an adc main and I mostly play ranked. After that, support really is my next most comfortable position and by a pretty good margin. Last night, and not for the first time, I got put into the support role 5 times in a row.
When I log in to LoL, I want to play ranked and I want to play adc. I'm perfectly comfortable playing support, but it's not why I log on. So when I get on to game for an evening, and I spend the whole evening not getting my "primary role," I get pretty darned frustrated. Particularly in ranked play where it means I'm not getting to play the role where I feel most able to carry a game.
In terms of overall game health, my concern is that beyond me, many players who may have in the past put "support" or "fill" as their secondary roles are going to quit doing that largely because it definitely feels like you're going to be put into support a disproportionate amount of games. And it becomes a giant feedback loop compounding the problem of getting support even more often for those who are still willing to put it as their secondary role.
What I'd like to see Riot do is put some code in place to ensure that your primary role stays your primary role. ie if you're secondary role gets selected 2-3 times in a row, your primary role is the only role matchmaking considers for the next 4 games.
For the time being, I guess I'm just going to have to put "mid" as my secondary role and hope to god I don't actually get it. Definitely not what I'd call an ideal solution... :-/