On one hand... by shaving our foreheads we are perpetuating unrealistic standards of male beauty imposed on us by the razor industry. What is objectively wrong with a naturally hirsute forehead, hairline down to the eyebrows as nature intended? And such as. It is a dispositive argument.
But on the other hand... if a bloke feels more intellectual and more enticing and more higher-self-esteemy by having his forehead depilated, it is his own business. We are not living in Imperial China with its sumptuary laws tying the maximum height of a scholar's scalp visibility to his success in the examinations! *
On the prehensile tail, a shaven forehead gives less purchase to moochers and parasites such as might cling to one's scalp in order to steal a free ride across the river. Also it removes any lingering doubt that one is a tanuki using his protean distensible scrotum as a head enhancer to pretend to a larger brain than one really has, i.e. "forging a head" as Keats would doubtless have observed.
* Pers. comm. Kai Lung story which I can't remember.