First, kudos to Tommy for a good, philosophical design topic.
Not that anyone cares, but here are a few thoughts.
As to the unplayability of a downhill bunker shot, most of the PGA Tour Pros I know have commented on their difficulty, especially when the green also slopes away from you, as would be typical with a bunker elevated about a green that falls from back to front. You may say, "who cares" because they are mostly concerned that a shot that they have the skill to play close to the pin may run away beyond their control. In their opinion, it goes a bit beyond rub of the green to an intentionally punishing design feature. And, they wonder if they have trouble getting it close, how would the average player fare? They side with Adam.
As to Tommy's more recent comment, creating an uncomfortable shot for the vast majority of golfers rarely makes sense. As mentioned golf courses are in the entertainment biz. Also, most golfers love it when their opponents have uncomfortable and difficult shots, but bitch when it happens to them! (Sort of like the difference between recession (when you friends lose their job) and depression, when you lose your job.
This has been a debate as long as I have been in the biz (1977) Ross was said to not use backing bunkers, although that may have been proven largely false. Mac used them for framing and artistic composition and they often work well in that regard. Somehow, when you have control of the golfers view (rare in other forms of architecture and landscape architecture) and the green is the ultimate target and (usually) the end of the view, back bunkers have at least a limited place. Given that only 8% of missed shots go back right, and 12% go back left, you cannot call them "high value" hazards compared to frontal and lateral bunkers. IMHO, a few per course where really necessary visually are enough.
As to their hazard, second level thinking would say, yes, protecting a back pin is sometimes a good thing. But in theory, if you have a mix of "Sunday Pins" they should be spread around like a tic tac toe board, and there should probably be only 2-4 back Sunday pins. And, George Thomas figured a long miss was a better shot than one that comes up short and deserving of a fw lie in many cases. Or, if you want to entice a golfer to play aggressively, leaving some wiggle room back tends to do that. Or, if you believe in a bail out area for most shots, including aggressive ones to back pins, they should probably be pretty rare on those tucked corners of greens that also have a huge front bunker and only 50-60 foot of target depth.
I can kind of see back bunkers where they guard the shot from the "wrong" angle much more than from the "right" side of the fw. When coming in over a frontal bunker from the wrong side, one typical good player strategy is to aim long and play more spin. A bunker back there, on top of a greater green slope right in front of it both allows and challenges that shot, if they happen to overcook it and play long. Or, with a bit lesser slope on the green facing that direction to make the shot itself a bit harder, (in addition to less green depth from that direction, tending to bring it in play more.)
As to "rear bunkers becoming the next gca trend" I think they were and now they are not, nor do I foresee it in any near term scenario. Just my experience, doing several bunker reduction plans. I have not been able to "keep" any back bunker I deem visually necessary. If they don't see play, most supers and pros say take them out.
Lastly, I think which architect uses them seems to influence this site's opinion of them. I recall C and C being praised for keeping the Maxwell bunker several yards behind 12 at Prairie Dunes. They reinstalled the fw cut behind the green to keep running shots running, potentially into the bunker. I would actually like to see the stats on how many players get in that bunker, 9.5 yards behind the greens back edge (I checked) even with the back center of the green kicking shots that way.
I have built a few bunkers several yards behind a green, usually on an uphill approach shot where I did want a bunker to help mark the green location.
Short version, while there are no hard and fast rules that should prevent them, of course, I agree with those who say it seems like for a variety of reasons, they just shouldn't be overly relied in a design.