Because it's hard. Here are all the ways this method can get you killed.
1) Drop your portal, everything works for a while, then you get Defile and Valkyr'd at the same time, causing you to port back into the Defile and wipe the raid.
2) Drop your portal, then move off to the side so that Defile doesn't hit your portal. Hee hee you're so smart. Oh shit the valkyr took you out of range on your circle. You die.
3) You pull off everything successfully, except now there's a Defile between you and the raid. Ok, so lets just run arou-Infest goes out and you die. Healers say on vent "He was out of range!"
4) You accidentally hit teleport. It's on cooldown while you get picked up. FML.
5) OK! So you drop your portal and watch the Defile cooldown. When Defile is up, you move to the side with it, then back again in time for the Valkyr. When Valkyr are off CD you reposition so you don't die to Infest and can still dps. You continue to re-drop Circle as you go. Then someone else fucks up and the Valkyr go in different directions, and during this Defile is dropped on your teleport while you are in the air.
And even if it were an exploit, they could make the drop command cause a 2-4 second stun and thus prevent this in the first place. Since they haven't, there you have it.