I often switch from tank to healer or reverse when entering Duty Roulette, so I pretty much wait about nothing compared to DPS.
However sometimes when doing weekly challenge to get a small boost when leveling another class, I go DPS on 3rd and 10th Guilhest.
To make the queue shorter on all the other queues, me and my buddy sign up together as tank+healer for those 8, which means istant start.
Instead when 3rd and 10th come and I go DPS, we wait for the two of that about all the time we took completing all the other 8 runs

Why am I saying this obvious thing, you may wonder.

Because since the day Duty Roulette came out neither me and my buddy EVER entered ANY Duty Roulette which had the "One Player" requirement, and that includes the elsewhere happily chosen Duty Roulette Guilhests.

Months ago, people on this forum were like somewhat superhappy that two-men teams like me and my buddy just couldn't enter the queue together. It's the lonely guy sindrome, which make them somewhat happy knowing un-lonely people at a disavantage to them.
They justified that by saying that number-wise, entering each one alone, would ease the queue process, rather than doing it together. Which, putting it that way, it's actually true.
However that cease to be true in the precise moment that Tank and that Healer (or even a Healer/Tank alongside a DPS, the useful role it's still there) don't get in that queue AT ALL, being it forbidden to be taken as a group.

I know many people like us who doesn't use any other roulette, because without the extra push of the increased bonus prize, we just don't see the reason to go to places like story raids or old trials. And we really don't. We didn't do till now, we won't in future.
Most of these kind of people are organized guys who rarely are DPS/DPS.
Just thing of married couples, "let's rush together!" friends and so on, at least one of them Always choose his/her role thinking about easing the leveling process and entering quicker in queues.
I'm not saying all of those people wouldn't absolutely queue alone, but it's a known fact that a part of them doesn't. Ever.

So, getting to the point, forbidding two players to queue for Duty Roulettes together, make that Tank or Healer or both that you could have to ease your queue forever unavaiable.
Making them being locked away from it a damage, for small as it can be, to everyone.