I know it's hard to understand because systems like Mt. Gox created a mindset in people that you are totally blind regarding your Bitcoins. That's not the case with multisignature-based services though!

Problem: People STILL don't know what happened to Mt.Gox coins. Whose hands they are now, when exactly they were transfered, what addresses the cold storage was on, etc., etc.
Solution: In a multisig service you can monitor your wallet in real time on the blockchain. If we somehow stole coins from you, you would know that immediately. We would have no excuse.

Problem: When a centralized service fails, often all users lose money. That was the case for Mt. Gox, inputs.io, Flexcoin and others.
Solution: You cannot steal from all users in a multisig service, unless Bitcoin itself has some fatal flaw (in which case we're all doomed). It could be possible to plant malicious javascript to the website, but that would be detected quite quickly and only a handful of users that were using the site at that specific time could be harmed. The "reward" is much, MUCH lower for a thief, so there's less incentive to risk a criminal act.

Sample scenario: let's say that at some point we have 10,000 BTC in our wallet (hint: we have *much* less at this moment). Most users only store few BTCs in their wallet, and only 5% of Bitcoins is in active usage at any given moment. So if we're lucky, we're get 500 BTC out before people find out. Is ~250,000 EUR worth risking jail time? For an individual, maybe. For a trade registered AG company with 75,000 EUR founding capital, not so much I think.