Encrypting Nexus 7 vs. screen lock
Basically, as screen lock secures your data as long as the data chip stays in the phone(as long as debugging is disabled [or enabled with no trusted credentials] and the phone isn't rooted), whereas encrypting it secures it no matter what.

Old answer:
It is possible to access some data via usb, however other data isn't accessible, plug your tablet into the computer and wee what comes up under internal storage, that is accessible (EDIT: none of it if the screen is locked). Of course given enough time a hacker in possession of your tablet could access all the data (as long as it isn't encrypted) (EDIT: for example as you suggested by removing the internal storage and reading it with a different device).
If your tablet is encrypted none of the data can be accessed without your passcode (if the tablet is switched off).
If usb debugging is enabled or you are rooted it is even easier to access the data.