The more players, the more available servers in your area, the wider variety of other players you'll find, the greater the opportunity for new experiences, and so on.Team Fortress 2 is a free download through Steam.
Another way we think of it is that there are a class of players who will never pay us a dime, for a variety of reasons. We're not upset by that, it's just a constraint we need to design around. The interesting problem to solve is how to make those freeloaders produce value for our paying customers. Obviously, getting those free players into the game is the first step to doing that.