The Dark Judges are classic horror monsters, but Dredd isn't a horror strip. They are probably so well-regarded because they are so odd (as Roger writes. As odd as a Dredd vs. Batman story: two very different genres clashing) in the context of a future cop drama.
Absolutely, their time was over when Bolland stopped drawing them (coinciding with a perfectly satisfying 'death' scene on Deadworld). All the other stories were creatively pointless. Judge Death introduced Death--he killed a few citizens, was zapped in Boing, end of story. Even Judge Death Lives had no real ideas: we've had Death. Hmmm, now how about four of him?). Necropolis went too far: what if there were umpteen Dark Judges?! Quantity over quality.
I do greatly enjoy Judge Death Lives, BTW!