I suppose it depends whether the song tells a story, in which case it's plainly silly to omit verses if that leaves the story incomplete. But otherwise, with traditional songs there may be many versions, and a short version may be just as valid as a longer one.

I suppose if it's a recently composed song, it's only fair to the composer to try to sing all of it, but some new songs are just too long for their own good and deserve to be subjected to a bit of pruning. Long traditional ballads are another matter, and it's only to be expected that they should take a while, but it needs to be in the right context, with a receptive audience.

If there's a chorus that people enjoy singing, they want to have at least four repeats of it in order to get into it. I have a theory that four or five verses are about enough for most songs (largely because I can't remember long songs very well). The are exceptions; if the verses are very quick and short, or if there's a story that will keep people listening, it can run to a lot more verses. Most songs take somewhere between 3 and 5 minutes - maybe less if there's no instrumental padding. Any longer than that and you risk people getting fidgety, or the MC getting impatient because he wants to fit in another spot.

I guess it comes down to the same thing as a lot of issues in folk music: if you think about what you're doing and why, and care about the decision you make, you won't go far wrong.

Marje