Unsupported. As Killanic said, this would simply decrease the RP of the car thieves. It is right that sometimes people just take your car to go to another car without even informing you about it, but some of the players use it for role-playing.
What I'd suggest is that the car (your own car) should despawn after more than 15 minutes. What if, for example, someone was with his friend away, and he was leaving. He wanted to give his car to his friend to drive it somewhere, but it despawn before he even can move away from the location. About state cars, I don't really support re-spawning them. I say that the state cars should not despawn at all, even if the player leaves for weeks. The administrators should do sometimes car-respawns, informing the players about it. As before, it was better than now, in my opinion.