I'm not comparing MP Rotators to Rainbird rotors. I'm comparing MP Rotators to Rainbird spray nozzles that the MP Rotators replace (i.e. they fit into the 1800 spray heads inplace of the nozzles).
As for something under 90 degrees, there's plenty of places where people have sidewalks that are at an angle other than 90 degrees to their house, driveway, etc. So there are lots of situations where you'd want to place a spray head in a corner, but that corner is tighter than 90 degrees. For these situations, there are adjustable nozzles that can adjust down to pretty much 0 degrees. So in those places where you'd otherwise use a nozzle set at perhaps 45 degrees, if you replace that nozzle with a MP Rotator, you will be watering more than your 45 degrees desired.
I've also got a situation where the landscaping is in odd narrow shape. One possible arraingment would place a sprayhead at the edge of this landscaping and require a spray angle of about 300 degrees if I only want the lawn watered. Again, Rainbird adjustable nozzles would allow me that 300 degrees. But the MP Rotors make me choose between 270 degrees, 360 degrees, or adjust the position and/or count of spray heads. (Because I've liked the idea of the MPRotators so much, I've just opted for 360 and water the landscaping with the lawn.)
BTW, I've now got a small set of these MP Rotators installed, so I've now seem them in action. Even at less than 90 degrees, they would still be rational. It's just that if the angle is small enough that you might not have both a "long" beam and a "short" beam at the same time.