I don't have a built in generator and I camp year round. Chances are in the real cold a generator will have problems starting so I would not rely on it as a back up unit. Starting batteries can be low or if it is run off propane and you have propane issues, or if it is run on gas and you run out of gas you are still stuck.
I use the on board propane heat, backed up by a catalytic heater with independent propane cylinders and as well as 110V electric heat to plug in if I should have power or if I am carrying the portable generator (benefit of the portable is that you can bring it inside to warm it up a little).
Are generators good for some things? Yes. Are they needed? No.
It really depends on what you are doing.