Cars with computers are well-known for being remarkably insecure and easy to hack. The only reason it isn't done regularly is because such "smart" cars aren't as prevalent as "dumb" cars. Think about it, these cars tend to have little to no security, so every moment spent even near one is a moment when a hacker could just activate it and do whatever they want to with it (limited by how integrated the computer is with the car), possibly even including driving it remotely...
It's not worth the danger. We have yet to see a platform that isn't able to be hacked, exploited, etc, and probably never will. The only way I could even consider tolerating such a vehicle is if it has NO wireless networking at all (no blue tooth, no 3g/4g whatever g connection, no WiFi, etc) and no third party was even capable of making software for it. If no one can make viruses that are compatible with it and there is no way to get any on it anyway, well then it is a lot more secure just by being completely closed.
At that point, well what is the point of having it? I just see these smart cars as a gimmick that may cost you your life, or the life(lives?) of someone else. Like I said, not worth the danger.