You have to have a payout scale and prizes to keep people interested. Obviously if a team is brutally awful it will not matter, but I usually award money to the top 3 finshers, each month's winner and each week's winner (except the AS week). You would be surprised how much a $5-10 weekly prize can motivate people.
That said, it is better to get a player to fill out the league than to play short teams and be at 8 instead of 10 or 12. If the person is not active and competitive, that is sort of their problem. As long as he is the exceptionand and not the rule it really does not matter, IMO.