TL;DR: By not enforcing any meaningful ratio of primary role games over secondaries, riot is driving a large number of support secondary into picking another secondary, diminishing greatly the available number of support players in the future.
At the moment, i am really concerned by the fact that there seems to be very little preference given to your primary role in dynamic queue. I am sure most people that tried for a reasonnable amount of time putting support as a secondary, no matter your primary, will agree that you become de facto a support main (and by a large margin).
I personnally intended to be a jungler main / support secondary, and given that jungle is also a sought after role, i expected to be able to play jungle at least 66% of the time, if not 75%. Unfortunately, i was picked for support again, and again, and again, while my primary role was selected less than one game in 4. While i do enjoy playing as a support here and there, i am much more effective in the jungle, and i felt coerced into picking another secondary so as to obtain my primary role a more reasonnable amount of the time.
Such complaints are nothing new, and a number of other thread on reddit already discussed them. Most support secondary in my friend list also came to the same conclusion i did and switched their secondary to something else.
However, i think it is important that Riot realise that this is bound to have a damaging impact on the future of dynamic queues. It is obvious that "abusing" support secondaries by putting them on support duty in a vast majority of their games will make for a smoother queue in the short run, as it increases artificially the number of available supports, thus firing queues more quickly on average. But in the long run, more and more support secondaries will feel pushed, like me and my friends, into picking a new secondary. At some point, there will be less supports available than if a more reasonable scheme had been put in place that allowed people to feel good about picking support secondary.
It is to be noted that this could be solved extremely easily. A very simple and basic idea, for example, would be to remember if you had been picked to play secondary at least once in the past 3 games you played, in which case you would be guaranteed your primary role in your next game. This would guarantee that, worst case scenario, you play your secondary once every 3 games, which seems like a reasonnable proportion, and would entice people to pick more freely their roles.