First off, it takes a decent amount of time from when you buy the vehicle to when it is spawned. Saying that it is immediate would be untrue.
Second, with said delay, it is very easy to be there before, or just as the vehicle is spawned, if you buy the vehicle from the closest terminal to the spawn point (inside the WF for GDI, and it varies map to map for nod). As an experienced player of the original renegade, I never buy vehicles unless I can jump in it in less than 3 seconds of it being spawned.
Third, and I only say this as a seasoned player of the original renegade, why would you even buy a vehicle without knowing FOR SURE if you can even get to your purchase without getting killed and having it stolen anyway?
This mechanic only seems to prop up careless play and punish risk takers who would dare try to expose themselves to steal a vehicle. Maybe if you wanted a 5 second lock to serve as an 'in case of tie' situation just as a vehicle is spawned, fine. But 30 seconds, as it is now, is just far too much.