Different people have widely varying reasons for taking dance lessons. Some want to compete, others want to be able to social dance, others just want to have fun and get some exercise. Some students want to learn a lot of patterns, while others want to learn technique.
There is no one right reason to learn to dance, except yours. However, I do have to wonder at something: the people who take group classes all week rarely go to the social dances, and we rarely see people who social dance in group classes. What are the former doing with their knowledge, and where are the latter learning to dance?
Learning any skill without practicing it is, in my opinion, kind of pointless. By going to class every week without practicing in between, you are cheating yourself because you will not learn as well or as quickly. Your body needs repetition, not only for muscle memory, but for building up the strength to do the movements properly. Plus, you are missing out on the whole reason for learning to dance--dancing!
As for the Saturday night only dancers, many of them have had a few lessons here and there, probably primarily the lessons before the dances. They know a few steps, but their lead and follow is weak, they have no style and their technique is poor.
Why learn technique? Having good technique is not about being a competition dancer or even a "serious" dancer. Good technique allows you to do dance more easily and comfortably with your partner and allows you to move around the floor more gracefully. It does not have to be hard and you do not have to spend hours doing change step after change step until they are perfect (although it wouldn't hurt). Just taking some lessons and learning some basics, proper footwork, dance frame and lead/follow will make a huge difference in your dancing.