Movement isn't the only thing demonic gateway is useful. If the portals are 50-70y apart it takes between 1.0 - 1.2 seconds to travel from A to B. Dots on you still tick but after testing a bit we found out that sometimes new effects/dots are not applied while traveling. So imagine the boss is doing his super ability and everyone jumps into the portal. Timed right you could avoid the effect. Like rogues vanish the stun in the professor putricide fight.
And there it is again, the thin line between "clever use of game mechanics" and "exploit"
But yes that is possible but also way to dangerous to rely on in my opinion.