Guadalcanal Hidden Rule Useless?

[Major Sylvester]

Guadalcanal rule specify any unit moving within 3 hexes of an hidden unit have 1/6= 16,6% chance to detect it. Therefore a couple of marine companies (8 units) moving will almost always detect an hidden Japanese unit 3 hexes away (probability of no detection= 0,8333^8 = 23,2% chance). Probability of not detecting the japanese unit 1 hex, 2 hexes or 3 hexes away =0,232^3=1,3%= negligible.
If at least 8 marines units are moving near a hidden japanese unit the hidden unit will (almost) alway be detected so why keeping this rule?

Shouldn't we roll for detection only once for each when it moves within in the 3 hex range but not when it moves closer. Or only once per hex entered. Otherwise the possibility of ambush is just too low.