Bikesharing is designed to encourage short trips within its service zone— even if you want a longer trip, you can just dock/undock a bike to avoid a fee (aka docksurfing). The escalating hourly fee is designed to keep the bikes in circulation, not as a charge you pay during ordinary usage— I've been a member of Citibike in NYC for over a year, and have never paid a fee beyond the membership.

With Zipcar, you first pay a membership nearly as expensive as bikesharing, then pay at least $10/hr.