There are a few caveats here that m0rtis doesn't mention, but we will: first of all, there's no way to guarantee whether someone is using an authenticator or not, so while you can make guildies promise, there's no real way to check up on them. Second, not all guild banks get emptied out due to hackers -- many guild banks get ninja'd by someone within the guild, and there's no authenticator that can protect against that. So having authenticators on bank members (or at least having them promise they've got them) isn't 100% protection. But it is something.
Not to mention that if you're in a guild where there is something worth protecting in the bank, you should probably have as much protection on your account as you can anyway. Guilds are going to have more and more riding on them as we move into the next expansion, so now might be an excellent time to make sure the ranks are covered as much as they can be.