The reason why the bayonet isn't triangular (anymore) is that it violates the Geneva Convention to make weapons that's designed to harm and not kill.
And a knife shaped bayonet is much more effective in a close/unarmed combat situation, since you can both cut and stab with it.
When it comes to bayonet training in the Norwegian Army we can still train our soldiers to use it, but it's not standard in the basic training.
It depends if we have a certified instructor available or not.
To be allowed to instruct in the use of bayonets you must be a certified unarmed/close combat instructor.
(wich I am by the way... 
)