QoT is the least-played DG. I didn't even know till this post that it has armor reduction. Reg and UB are the most-played. See where the discussion goes? There surely other good combos.
UB: spit (and for some reason no one's mentioned how badly a multi spit can wreck towers, allowing easy map control), grab, slowing (there's only a couple items/skills with which you can hope to get out of their range once they've started hitting you)
How is this harder to escape than Erebus+UB? Erebus is just as fast as UB and can apply a brief, heavy snare. Also night walkers wreck towers much more reliably than spit.
Reg: range (wreck towers pretty well without taking ANY damage), snipe, mines
Double mines? Sounds great until people start just using potions in your face, that's what I do when there's no interrupt present. Also how are double mines more effective than spit+mines or any other combo?
Sedna: Pounce Pounce Heal Heal Pounce Pounce Heal Heal walk out of your base over your dead body. I see this one alot. If they get in range of you, whether or not you have a partner in the lane with you, you're gonna die if they chose the right items
I'd take Sedna+minion swarm erebus over double sedna any day. Sedna+UB could stomp it too, and I guarantee I'd mop the floor with double sedna as QoT+Rook too.
Rook: 2x blink, Boulder Roll, 2x Hammer Slam, 2nd Boulder Roll and slap if needed, towers all over the place to teleport anywhere needed (I haven't seen this, but I think it would be hilarious to gank people like that)
There's a good reason why you haven't seen it.
People keep saying double Regs suck, but I think they've been playing against some of the many noobs that don't even watch the DGs in the other lanes to snipe them. Most of the Regs aren't very good, and it's usually GG if one of them is bad.
Double reg is annoying until you have priests or monks. After that point it's no better than many other combos.
No one reasonable is arguing that doubles suck, they're arguing that doubles are no better than mixing DGs.