So OpenGL rendering is just useless here.
Not necessarily. There are many obscure problems under the hood with the Doom rendering engine, which can cause it to act funny under certain circumstances. To fix certain bugs and add new, vital features can be a major pain in the ass for a software renderer that is as old as Doom's. A new renderer could be very helpful to solve such problems. Take for instance
this very famous Odamex bug. Note that this bug not only exists in Odamex, but in
every single Doom port that uses software rendering at high resolutions ever. ZDoom has it, ZDaemon has it, Eternity has it, Legacy has it, they all have it. Months have passed, and the team has been wracking our brains from here to kingdom come trying to come up with a solution, but so far the solution has been elusive. A similar problem has been found by the developers of Eternity where lines above a certain length could be rendered inaccurately in the software renderer, and it was finally decided that
it was a fundamental problem with the Doom renderer, and had no conceivable solution. None. No way to fix it. The only solution is to write an alternate renderer.
The fundamental problem here is that when you think "OpenGL", you automatically think "GZDoom" and "Doomsday Engine", which you don't necessarily care for. I understand, I prefer to use software-rendered engines myself. However, have you considered that OpenGL doesn't
have to necessarily be "colored lighting and special effects galore"?
OpenGL allows easy ways of doing graphics intensive things for purpose-built GPU's that were a huge pain in the butt 13 years ago when the routines had to be built for a general purpose CPU's. What if an OpenGL renderer was written that didn't filter textures, scaled light and darkness like the original renderer, and for all intents and purposes looked exactly like the original engine, but under the hood was much easier to understand and maintain? If written carefully enough, you might not be even be able to tell you were USING the new engine (except you wouldn't run into mentioned problems). "Impossible!" you say. Nope, I've seen screenshots of denis' ogl_hack branch of Odamex, and it's so close to the original software renderer, it's scary.
I'm not saying that Odamex is going to have an OpenGL renderer (or not, for that matter, it's still in the experimental stage at the moment). However, please don't misrepresent what an "OpenGL renderer" is.