The sole reason that I would choose a pay sportsline league over a free ESPN league personally is minor league functionality. I play in one CBS league, and CBS's interface and look are inferior. If I were the commish of this league, I would move to free ESPN leagues in a heartbeat, and I would go through the extra trouble of managing the minor league portion of our league on the message board, rather than charging $10 per guy just for the minor league functions.
If I am paying for a league, I expect to get something far above and beyond what I can get in a free league. I feel that CBS has some positives, but it gives me nothing so special to where I would pay if given the choice.