Richard Muscat:

Free rarely works, and all the times that it doesn’t, it undermines entrepreneurial creativity, destroys market value, delivers an inferior user experience and pumps hot air into financial bubbles […]

Free does not push you to create something evocative that users and customers are willing to commit to in the long term.

Free absconds on the entrepreneur-customer commitment: by asking for nothing you also promise nothing. Both parties can walk away because there is no relationship. On the other hand by asking for money (or some other form of commitment), however large or small an amount, you create a self-imposed drive to produce creative and valuable products because not doing so would mean letting somebody down.