Nowhere on Banzai charge does it state your opponent cannot defencive-fire against it.
If the defencive-fire is succesful the banzai-charging unit can no longer gets to attack unless it was stopped in the same hex as the target (which would seem a rather foolish thing to do).
It would be completely up to you if you want to House Rule Banzai charge, just be aware those are the actual rules if you ever play in a standard/tournament game. For the most part these changes were made so BMWs couldn’t Evel Knievel across 5-7 hexes to knock out tanks, etc. As it doesn’t work on artillery I don’t believe there’s any other units a 2+ charge distance would apply to.
Here is the location of the FAQs and errata pages: http://www.wizards.com/default.asp?x=ah/aam/ah20050914a