Do you think it was a great feature that older operating systems like MacOS would allow you to write directly to memory rather than through a protected memory manager?
That's a completely inaccurate comparison, because the whole purpose of the DRM and DRI is to provide protection to applications' video memory and enforce cooperative use of the GPU without adding any unnecessary protocol encoding/decoding and dispatch like indirect rendering requires.
The whole reason that AIGLX is so important is because indirect rendering is required for an OpenGL-based composite manager: with direct rendering, the composite manager cannot access the video memory that the other applications' windows are rendered into. By putting all of the actual hardware interaction in the X server, the X server can use it's "root"-like video memory privileges to do the compositing. The compositing manager's job is simply to tell the X server *how* to composite things, but not to actually perform the composite operations themselves, because it doesn't have the video memory access permissions to do the compositing. Only the X server does.