I do it all the time. Looking out for traffic but inside for flying. In fact, by not looking outside the majority of the time, my brain likely has no idea what a proper outside attitude for straight and level flight is in the current conditions. I use solely the AI to maintain a proper attitude that will result in desired performance.I think it would be impossible to look outside of the front window of an aircraft in VMC and not subconsciously mentally integrate the sight picture one is presented with, with the information from the instruments - and thereby no longer be flying only by reference to the instruments. At least, I am not Zen-master enough both to see the horizon and not see the horizon at the same time.
Not quite true. The Instrument Rating Flight Test Guide says the following:
An Instrument Proficiency Check (IPC) is to be conducted in accordance with Advisory Circular (AC) 401-004 - Second Edition
There is no mention of the vision limiting device in the aforementionned AC with regards to the conduct of the test itself (although it is mentionned for proficiency flights in preparation for the IPC