Should conferences do early rejection? A number of conferences have moved to a two-phase review system, in which for example all papers get two reviews, and then any paper that has received at least a "weak accept" advances to get two more reviews.  In the conferences that use this, I'd say that typically 1/3 are eliminated early and ultimately receive only those two reviews.

In general, conferences stick to a single notification deadline.  If they opt for rebuttals and offer a rebuttal after the first phase, authors may withdraw upon seeing the writing on the wall.  But if there's no interim rebuttal, authors don't find out until the notification date that their paper was dead perhaps 4-8 weeks earlier.  

I'm familiar primarily with computer science systems conferences.  In fact, I have served on a fair number of program committees, chaired a few conference TPCs, and serve on some steering committees.   I'm curious whether the two-phase review system is used in other fields, and if so, do any release the rejections early?  And of course I ask because I wonder if I should advocate that conferences I have any say in move in that direction.
I'm in the health and social sciences and I have never attended a conference that has used this format for submissions. This seems like a lot more work for conference abstract reviewers and program committees (all who are volunteers). 

I have seen a movement towards "late breakers," where you can submit an abstract for a subsequent round of reviews, often after the initial acceptance/rejection notifications are distributed. You typically are not able to re-submit a rejected abstract, but could potentially revise or (ideally) propose a different project for the conference. I guess there are pros and cons of each approach.