I (at least) debug my shaders with the logs spewed from GL.
Is it required to debug shaders? No.
The specs say a lot of things. Implementations do a lot of things. Unfortunately, neither is a subset of the other. A good portion of my time goes to dealing with bad behavior from non-spec-compliant implementations. I'm not excited to invest even more time in this just so that error messages are prettier. (They seem sufficiently functional at present to me)
If it's so easy to do, why not have a library which does it. That would at least halve the development time required. Submit bugs to browsers if they emit error logs that don't match the GLES spec.