The reason to become a paying member, to the best of your ability, is to support the institution as a shul, to be part of a kehilla. It cost a great deal to maintain and operate building, and a kehilla. Few rabbanim are making it big, but their salaries must be paid on time, siddurim must be purchased and maintained, the building must be cleaned and upkept, etc. Purchasing a seat is just one way of being part of the kehilla, and having your name on the seat does not mean that your neighbor’s guests cannot sit there if you arrive late. As a kehilla, you share and share alike, based on the standards and practices of the kehilla.
Shul membership is not like buying a slice of pizza. You buy in because you believe in it and you are part of the group.