Yep, it may look a little crappy, but, you're right: for 10 lines of code, it's hard to justify not adding it.
I knew z-shiftting mlook should not be hard for implementing, but never thought that full implementation is about 10 lines of code. I added mlook to software mode to prboom-plus this night. Sky stretching code was taken from ZDoom.
Now, it gets a bit more tricky to do some other things, like weapon aiming. You *could* leave it original, but, with orignal code, you can see the monster, but not be able to hit it, with extreme mlook angles. Of course, that happens without mlook sometimes as well :)
Another issue is the non-vertically-wrapping sky. I seriously wish someone with artistic abilities would make a set of super-tall skies that look original. :) Please! (or did I miss them?) I know there are some slightly-larger ones out there (240 pixels tall vs. 200, but that's not enough.) Of course, the port must have 'tall wall' support.
One last thing: Killough's renderer optimizations and mlook don't quite jive - I remember having to modify some Killough clipping code to prevent sprites from disappearing. Like if you're at the top of the stairs in E1M1, I had shotgunners disappearing below me intermittently, when they should have been visible. I don't have the code in front of me now, so I can't tell you what I did to fix it - but, just test it - maybe it's not a problem for you anyway. But, if you fix it wrong, Doom wants to render sprites that are way above or below the screen boundaries, and it's caught much later in the pipeline, so to speak, right before rendering, which slows things down a bit (per row clipping vs. per-sprite).