I think the issue is, to do anything useful with a witness, you need the active key. But once you have the active key, you're now able to move funds at will.
Unless everyone transitions to "yourname.witness", then STEEM needs to have a new type of key. A key which lets you change your signing key, and publish a price feed, but nothing else. Right now, anyone helping out with your witness will often end up needing full access to all of your funds, as they need your active key to change the signing key in the event of a failure.
I agree that a witness key would be beneficial. However, it's not too much of an issue currently.
a) Collaborate with people you trust.
b) Since all rewards are paid in SP, you always have 7 days to react to any moving of funds.
c) You can set a power down withdraw route to a different fund account entirely, so the witness account never sees any liquid funds. Should someone change the withdraw route, you once again have 7 days to fix the situation. That'd be enough of a deterrent so no one tries anything funny.
d) Multi-sig. I don't really know how that works, but I know it's there on Steem and @steemit uses it.
@liberosist I too just voted for you as witness and I appreciate your ability to express yourself about this topic.