In order to determine whether or not it would be helpful advice, we’d have to see who the advice was given to.
There are a variety of reasons why I think it can be helpful to think of the non dominant arm during a backhand, just like we should think about the non dominant arm during a forehand, overhead, slice backhand, volley and on the serve. It would be ironic to me if we were to overlook the non dominant arm during the only stroke we actually use it to strike the ball.
There are many players, if not all of us, who tense up at contact and as a result sometimes don’t follow through as fluidly as well as we would like.
If someone is not following through, there are perhaps three or four different cues to get them to do so, one of which being “show the butt of the racket” to the far fence. Of course we could do that over and over with both hands on the racket but if I were to see someone let go with one arm after contact (as a practice method) in order to achieve the feeling of being more relaxed and in turn point the butt of the racket to the far fence, I would much prefer to see them practice following through with their non dominant arm than the dominant one.
I think there’s a reason that the finish of a one hand backhand is different than that of a two handed backhand. In addition, the two hand follow through is much more similar to a forehand than that of the one hand drive which is right hand (for right handers) dominant.
As for the topic, to simplify things, the type of grip we use on our two hand backhand will literally play a huge role in deciding which hand is actually dominant. The further your right hand is to the top bevel the more “important” or “dominant” it becomes. If one rotates more towards an eastern forehand grip or even semi-western (which is never advised) the right hand becomes weaker and therefore less important in generating (even) power.
With that being said, I’m on a continental grip on my two hand backhand (which is closer to right arm dominant) and still consciously use the left arm during my stroke.