I've never fought a Njerp with a dog, but I have fought quite a few Njerps (and robbers). One very important thing to keep in mind that just because one is a pushover, the next one doesn't have to be. Some are very difficult to deal with.
The problem with a dog companion is that it may attack from behind while you attack the Njerp from the front, and you can't defend against attacks you can't see coming...
Njerp + dog should normally be less of a threat than two Njerps, though.
Also, dogs don't have armor, so a lucky hit can kill it with a single stroke (Njerps have done that to a number of my characters' dogs...).
Finally, UrW fights are dangerous. A single lucky hit to the eyes can easily end a character.