The other keys can do everything but change keys (transferring funds, voting, etc.). The main thing that protects the value of an account as long as you have the owner key is the powering down process taking two years, and one week for even the first payment. If someone were to take over your account (via the other keys) and for example start powering it down or using it to vote on their own posts, you could recover it with the owner key (by changing the compromised keys) and limit the damage.
However, if you have liquid steem or steem dollars in your account those could be stolen immediately with the active key. The memo key could be used to see your private messages.