"...except where it is necessary..."

I was reading an introduction to alto clef for (I'm guessing) composers and stumbled upon this statement. I nearly fell out of my chair laughing:

"...but the extreme upper notes would preferably be written for the violins except where it is necessary to carry the viola into the high register to preserve continuity of tone color in a passage or where the peculiar tone quality of the viola A string was purposely sought."

Yes, indeed. Unless an extraneous situation dictates otherwise (a.k.a. the peculiar tone quality of the viola A string), let's leave the high notes to the professionals.