This is what I am saying:
Since all the accounts are locked to their own folder, the .dll will only be able to access their own folder (This assumes you are using Dimitri security method or TCA v2, user per service method, which I both highly recommend). So yes, I can go block .dll, which will inconvenience everyone, but the security it provides is FALSE. With srcds, the exploit which allows .dll will bypass TCA restricts, as it gets written by srcds.exe, not TCA's FTP or File Manager.
What your saying is, what if the client uploads the bad .dll? Well then you just hacked yourself as you won't be able to access anything outside of the game server root.
What if the someone uses the exploit? Then they get to hack the client's server, and they would probably only end up with access to server files and the rcon password as no one keeps there billing and control panel details in a text file on their server. To prevent this problem, the clients can disabling uploading, and enjoy a safe server.