Orbital Explorer currently hangs the GPU when run with INTEL_SCALAR_GS=1. This would need to be fixed before we could consider enabling the scalar backend by default.
Also, it generates awful code: something on the order of 3x the number of instructions, along with spilling. The vec4 code does not spill.
For reference, here's my fork of orbital explorer that uses waffle and builds on Linux (the original was Mac OS X focused, and now unmaintained):
We would also need the INTEL_SCALAR_GS=1 to use MaxImageUniforms. A couple of the Khronos CTS cases for OES_geometry_shader exploit this.
INTEL_SCALAR_GS=1 is now default. Orbital Explorer no longer hangs, and I tweaked some things to reduce the number of spills significantly. I think this is as good as we're going to get for now. The application author has also declared the project dead.