In what aspect? It being rooted doesn't give the stealer access to any more info than if it wasn't rooted. Always best to secure your phone with a good pin and install a security app that allows remotely wiping your phone (lookout, AVG etc ) if you are concerned of sensitive data on your phone.
Always a valid concern. As someone who has experienced 2 laptop thefts in my lifetime, I would seriously caution any user to consider the consequences of storing sensitive information on a portable device.
I'd recommend keeping sensitive data stored on one machine and keep it encrypted (preferrably a desktop).
I understand it is convenient to use mobile banking apps, etc from portable device like a smartphone, but it's so easy to have something as small as a phone or tablet become lost or stolen. My personal philosophy is DON'T DO IT...if you can live without that stuff on your portable then avoid it and you have less to worry about.
If someone has physical access to a device and are savvy, they can get your data. And who knows how much time they've had the device before you discover the theft and initiate a remote wipe.
If they could manage to load something like clockwork recovery, then AVG, lookout, screen locks, etc.. all become a moot point. They could use clockwork to dump a backup of your whole device. Encrypting your data would be a better route than relying on easily defeated security like a screen lock.
Rooted or unrooted, if you're going to store sensitive info or engage in risky activity, then you have to be extremely diligent. Forget to encrypt one file and you're at risk. Regardless of the what type of device it is.