some times I tried to use rect lights and I run in the issue, that I got a sharp light-nolight-border over the scene. I send Thomas Schander and image from a current project. Here a simple test.
I found, that the bug appears if the lights are scaled as long thing light sources. Here two tests in comparison to Vray for Rhino. I hope a fix is soon possible, since it would help a lot. Often I have long thin wall washer lights and emitters works not perfect. So, I placed rect lights over the emitters and I got direct light to the scene and a much better intensity control. Only the wrong light distribution is a strong problem.
thin light - wrong look
quad formed light - light distribution looks quite good, only it could coded a little bit softer at the end. So it could help to avoid unwanted light borders and the rect light could be used more often as offset light source to enhance the lighting of emitters (that's I did often at Vray too).