It is necessary to have a generator in a 5th wheel, I know some have them some don't. We are looking to go to a 5th from our class a and we really don't use ours a lot now.
If they were free I'd have a generator just because, but if I had to pay extra for it, not very likely. We have one because it came in the toyhauler. I think I've actually used it a couple of times. And those were just because it was there, if it hadn't been installed it would not have been missed in the least.
Mostly it's a pain because it's just one more thing to maintain and fuss with.