The main reason people would like to secondary as Support but don't anymore.
like myself in the past(and sometimes present), I only really enjoyed one role and if i couldn't supp was usually available and fairly enjoyable form time to time.
An example would be a jungle who doesn't enjoy last hitting minions for days, would obviously like to support when they can't jg because that's the only other role not dependent on last hitting creeps.
But since queing as supp 2ndary gives u supp almost every time, it's just no longer an option for non supp mains.
I guess you could say this thread is a response to all the threads talking about "just don't que supp 2ndary", and similar topics.