Google doesn't really need to throttle attempts at guessing Drive URLs - they are long enough that it is totally infeasible for anyone to guess them. The guessa

Google doesn't really need to throttle attempts at guessing Drive URLs - they are long enough that it is totally infeasible for anyone to guess them. The guessability of these links is not the weakest link in finding these documents - it's that they can easily be shared around and you don't know who has the link and who doesn't. This may be okay for your usage - it's a tradeoff between usability (it's easy to share such a link with your friends instead of granting each one permission individually) and security (your friends could forward the link to others without you knowing, which they couldn't do if you'd only granted them the permission).