When he says "not practical", I think he's referring to how putting wood on an AR would negate it's role as a lightweight rifle. Not to mention, the polymer commonly used on ARs is very durable whereas wood could be more prone to rotting or burning or other deteriorative outcomes. This, of course, is dependable on the type of wood and quality of workmanship - as it could be as or more durable than the polymer. However, lightweight it would definitely not be.
I'm only trying to interpret his intentions behind the words. I may be wrong, but what I said seems the most logical.