Let's say you're playing in a tournament, right? You know who your next opponent is, and the map it's being played on.
You're feeling a little bit uncertain about what possible tactic your opponent might use on said map, so you search for his/her username in the replay vault, and voila... hey, here's a replay that happens to be on the map in question. So you take a peek. And see what your opponent is doing.
Sounds innocuous, everyone does it, right? Even professional sports team analyse their opponents' tactics, strengths and weaknesses.
The difference is, professional teams have the option of practicing in private, away from prying eyes.
As far as I can ascertain, if you play some practice games using the FAF client, you do not have the option of privacy for your practice session. You can't try out stuff and maybe settle on a strategy without it leaving telltales which might be used against you.
I consider it to be a serious shortcoming of the FAF client that you cannot practice in private. Your alternatives are to create sock accounts, or set up a lan game, or otherwise jump through whatever hoop you need to in order to keep your identity on the replay confidential.
What the client needs is a vault veto option. Very simple - the host of the game can tick whether he/she wants the replay saved to the vault.
JOB DONE.