There is no real difference between a private and a leased car.  They're both used by one person. The fact is, carsharing is a much bigger threat to the leasing business (where people don't own a car), and that's why it's desperate and willing to pay so much to wipe out the competition.

It's not all about the money. Your members count too.

If you get swallowed up by the multi billion dollar leasing company, they'd just hold you back in favor of their standard service. They wouldn't do such a deal if it didn't promote them much more than it promoted you. They want to delay their end by bringing you to an end (or a halt), before you get too big and they can't stop your spread. They have so many cars they could beat you out in a fair game with their own carsharing brand, but they didn't. I wonder why?

There is no reason for you to be one company with them or owned by them-
They know it, and so should you.

If you really need to get more cars (without paying upfront) you can just hook up with RelayRides or other p2p car sharing sites. This way, any car can be a Zipcar.  Doing as you plan would irreversibly hurt your public image and betray your loyal crowd.