Googling around suggests that this involves the zipcard - if you don't use it to enter you can't start the car. There's no real reason that other companies can't do something similar.

Also (and perhaps I shouldn't say this) I often see additional zipcards in the trunks of zipcars - by the spare tire - presumably for use by service personnel.

I think you're right that outright theft of the car is unlikely. But the advantage that Zipcar has over this p2p system is Zipcar personnel can keep a log of everything related to a vehicle - the condition of all of its parts and a full inventory of its accessories. This also allows Zipcar to make final decisions about who has caused damage or theft. In a p2p system, there is no central authority to adjudicate these issues - which requires relying on the word of either the owner or the renter to decide who is responsible.