I learned to support so I could second-role it than get auto-fill protected and be able to queue up my true secondary w/o risk. I haven't been auto-filled a single time this season in ranked. However, that is beside the point. I'd bet that ~90% of people would prefer a 5-minute queue or even longer and not risk being filled than to either lose 3 LP and go on a timer or spend 30+ minutes (including champ select, loading and post-game) and lose whatever one's LP loss is. So I agree with you when it comes to a game to game basis but on the other hand, it creates cases like mine which encourages (or forces...) players to learn a role they'd rather not.
At the end of the day, I don't have a solid opinion one way or the other. Both sides have their positives. One being that you won't be placed in a role that is a near certain loss unless your team can pick up the slack, the other improves queue time and potentially the state of the game by increasing the player base that can support at a higher level. I think my approach is optimal for both myself and the game. It's not like learning the role is extremely difficult and it ensures (unless one is unlucky enough to get filled while having support as secondary) that one will only need to know two or three roles.
There is a problem and the only solution is auto-fill. When I played WoW several years back, I recall getting 20+ minutes queues in LFG when I wasn't on my tank and in a game that doesn't have something to do during the wait, a queue time that could be as long or even longer than a match itself would almost ensure the demise of the game itself.
[sg-lux-2]