I must admit that in that situation, if I had taught students from textbooks that ruled the construction unacceptable, I would probably have argued the point myself.
I tend to agree with you on this, but I believe that the emphasis should always be on clarity through rules, rather than on the rules themselves. However, I understand that such a distinction can be difficult to make clear to a bunch of unruly youngsters.
j.c.…<blush> 