Yeah, I had set 5 or 6 of my favorite cars to "Share with Friends" and when the servers were down this past Saturday I was unable to use my shared cars. I wasn't happy about it either because I needed to use 1 or 2 of those cars.
There is no real point to share the car other than no one being able to use it. From now on I will only share photos of the car!
It would have been a better feature if this function allowed a friend to check out and use the car for an event. That way you wouldn't need to gift the car and hope to get it back if you weren't planning to giving the car away.