It's perfectly reasonable to carry business data on a mobile device that you wouldn't want to be on any 3rd party web server: All your contacts, dates, ... need to be on your mobile. Contacts at least. It's quite pointless to carry your mobile around without having the numbers in its memory, isn't it?

There's a risk of losing such a device, yes, but that's about it. If you do, and if somebody else gets access to the data, you'll be held responsible. (Most important: You notice when it's lost.)

Using whatever service on the web is differnt though:
You can safely assume somebody will have access to your data. You don't know who, when and how often, though. That's not a risk you put up with, it's something you know will happen. It's like driving a car (knowing you might have an accident, but chances are low) vs. jumping onto the highway right in front of a speeding truck.

That's why I'm not allowed to use any kind of syncing mechanism except via USB with my mobile phone.