Dedicated, preplanned rental can be done much more cheaply from conventional car-rental places. If you're on a business trip your business probably has a deal in place with one of the big companies, and you probably want to just have a car for the whole trip and not worry about it - especially since you're going to be expensing it anyway, and neither you or the expenses department wants the trouble of figuring out exactly how many hours of car rental is reasonable. Zipcar's great advantage is for unplanned, one-or-two-hour rentals (often because you need to transport something bulky, which a business would have specialist ways of doing), which is far more common for private individuals than businesspeople.

I used it infrequently as a consultant; my company actually introduced me to the service -- but we used it to supplement traditional rentals, not replace them. Got to run out to play pet engineer for a scoping meeting? Grab a zipcar. Stuck in the ass end of the midwest for a two week engagement? Traditional rental.

I don't think they are leaving it on the table, I think their schedule of fees means they are carving out a spot that the rental companies totally ignored.

Actually, here in Belgium my carsharing service (Cambio) tries very hard to aim at businesses too, although more for traffic between work and home/station than business trips. Since most private use is in the evening or at the weekend, this is an obvious way for them to crank up usage per car.