I would say for MOST situations a hood is necessary. But there are scenarios where you don't need it. I live in Colorado where the air is very dry and I brew in a 2500 square foot basement. I don't use a hood, only a fan positioned above and behind the boil kettle to push steam out into the center of the room, then another fan pushing out a partially open window with another window partially open on the other side of the basement. I don't have any issues. I monitor the humidity and it goes up from about 20% to 30% on an average brew day and drops back to normal very quickly. I never get any visible steam build up or any condensation on any surface.
All that being said, I would never recommend going without a hood unless you had a scenario similar to what I have. Even then, I'd rather have one than not, I just happen to know that my setup is temporary in its current location so it didn't make any sense to run the ducts, cut the wall for the vent, etc.
Anyway, food for thought...
Dan