[2020] is killing zombies on horde night worth it?

Every zombie killled is a chunk of exp in your wallet. Also, just ignoring zombies could be pretty bad for your walls, as when numerous zombies clump up in one spot on your wall, they start doing magnified damage and can pretty quickly shred even steel bases.

And as 9029 says...If you're not killin zombies on the single most impactful night of the week....what are saving ammo for?

Also it is possible to exhause the zombie respawns- It gets increasingly harder the further into the game you are, but, you Can kill enough zombies that the horde night ends early.

It's a fair question, and one a min-max type player would consider. It certainly seems possible that you could stand on one side of a door or wall with your nailgun out, and continually repair it, keeping pace with the damage the zombies are doing. You could funnel the zombies into a narrow corridor or more complex path in front of this intentional weak point, so as to minimize the effect of bunching up for extra damage.

It may not only be viable - it may be the case that this is cheaper than using (ranged) weapons to fight them. Ammunition is one of the more expensive items to amass in the game, whereas with this approach the only consumable items you need to acquire are stacks of repair materials.

So I haven't run the numbers, but yes, it seems like it could be worth it, in the sense of being an equally if not more efficient use of your time and resources. And after all, efficient use of time and resources is how you survive. Whether it's worth it in terms of spending your game time doing this depends on what you consider fun, as others are saying.