But when you're doing a group PVE mission, they don't mind you running around in circles?
I don't think you are understanding him really. In a typical PVE setting, the only way someone will know if you are mouse turning is if you spin around quicker than you would by keyboard turning. You wouldn't be circling your enemies or anything during a fight(unless that was an advantageous mechanic) I think he's just saying if you practice strafe/mouse movement while you are playing pve it will come more natural. If you go to www.taugrim.com
... that player actually made a decent guide about keybindings that may be beneficial for you.