By locked I mean a phone that has been locked with a standard PIN code, password or pattern.
I'm not meaning to break into somebody's phone, merely wondering to what extent applying any of the standard screen lock methods to my phone would protect my data in case of theft.
In case a locked phone can easily be rooted, I assume the lock can then also be bypassed, and locking the phone is then really of little value for protection (since rooting techniques are common knowledge these days), and I would then have to consider other protection mechanism, such as encryption.
Please keep your answer concise, and don't give general security advice. I just want to know if there are known (widespread) methods to root locked Android devices.
Note this question is with regard to Android phones with factory default settings (i.e. no developer options, USB debugging enabled).
I'm specifically interested in Nexus devices running Android 4.4 KitKat.