Night Goblins with short bows was a good idea. For an extremely cheap point cost, you get an additional deployment, which is always good for an army that wants to outdeploy their opponent. Of course they have the ability to deliver fanatics and send them into the enemy lines, and they can provide a speedbump for enemy monsters or cavalry, who will get stuck taking down their ranks for a couple of rounds as long as the Warboss is within shouting distance. In a pinch, they can even provide ranks if combo charging with something hard hitting, such as a Giant.
With the new rules, it's now possible to deploy them 10x2 and have them fire 20 shots with a potential range of 22" (includes moving forward) and they can even make a swift reform if they bring a Musician.
In the new army book, they get an additional bonus, as the second spell of Da Little Waaagh! is called the Gift of the Spider God, and gives a unit poisoned attacks. It's a perfect little spell to cast on your bowgoblins in the very first rounds of the battle, as the poison allows them to take out enemy fast cavalry or other annoying little units. If they move and fire at beyond 9", they'll hit and automatically wound on 6's, and for 20 attacks that can sometimes mean quite a few. If they remain still while shooting, they can do the same to enemy skirmishers. In the later stages of the game, you'll probably want to give your speargobbos the spell instead, but by then the bowgobbos will have done most of their work.
I always try to bring a single unit to every battle, since their uses are so many. In most cases, they will be my only missile unit (not counting artillery). Even if they don't do anything useful in a battle, it's such a tiny point investment that it probably won't hurt you anyway.