[interpol_p]

I don't get it, though. How can the reason be due to benchmarks / performance seeking?

The driver simply has to zero the buffer when the new OpenGL / graphics context is established. It's once per application establishing a context, not per-frame (the application is responsible for per-frame buffer clearing and the associated costs). At worst this would lengthen the amount of time a GPU-using application takes to start up and open new viewports, but that hardly seems like it would matter or even register on any benchmarks.

[c0n5pir4cy]

The thing is it probably not once per application. I'd imagine using multiple frame buffers in an application is actually quite common and could change quite often while an application is running; especially in complex applications like games. It's probably not enough of a hit to really justify not clearing the buffer but it's enough to make it noticeable in the benchmark race.