Auto Part Stores would be a good idea, both in terms of realism (I live in the middle of nowhere, and my town still has two of them), and convenience. However, vehicle parts are only useful in a very limited set of circumstances; replacing destroyed components, mending faulty engines, changing tire sizes. Since the uses are so limited (once you get a vehicle working perfectly, the odds of needing another engine component are pretty low) the work involving creating an entire building revolving around those components wouldn't be worth the gain from it.
It's not a matter of whether it's a good idea; it's a matter of whether the devs have the time to work on something so small, and they're already pretty busy fixing things that might break the game (for free, I might add), and trying to make it better. Maybe you could add it as a mod, though. If the building has a chance to spawn a welder/acetylene torch, or a vehicle welding rig, and welding goggles, I'm sure it would see plenty of traffic.