That actually has been brought up before and I like the idea. Some people have placed a cryptic code on their cache (in the past) and a person had to send that via email to the owners. For some reason, I thought that they didn't allow that anymore.
If the site did it, you wouldn't have to depend on other people to check their email - it would be like a password - you have to have the password to log a find. Oh well. I'm a newby - haven't had a problem with this yet - I ordered some caches and will be placing them sometime this week.
hmmm~You know with that in mind, why couldn't a person write a TB or geocoin number, say on the lid, and impose an ALR (additional logging requirement) saying that when logging the cache, you must also discover the coin/tb number on the cache. (Do not place the actual coin/tb in the cache) Log the cache and discover the number wouldn't be a cure all for those who would ask a "friend" for the number.
<The above is just an "out loud" thought>
IMO if its that dadgum important to 'em, let'em have it. lol.