WWI was the last major war where H2H combat occurred on a semi-regular basis. Both sides would Raids each others trenchs and observation points at night in order to secure prisoners.
From what I have read in soldiers experiences in fighting in the Trenches a sharpened shovel was considered a much better weapon than a bayonet.
Thats why I said if your out of ammo. If your opponent still has ammo you are screwed regardless if you use your helmet, your shovel or your high end fighting knife. He will send you one or two of his dirty little helper´s while he is wondering why you wave your shovel over your head .Always remember Murphy's First Law of Bayonet Fighting: The one with the bullet wins.
Same goes with your first aid box in your car or when you go hiking. You wont need it most of the time or hopefully never but you would be thankfull to have it if the "worst case" happens.You might hear a story about some guy who used a bayonet in such and such situation not too long ago but the chances of that sort of thing happening are so incredibly remote that simply the bayonet isn't worth the weight and space it takes up.
There are several recorded examples of the AIF routing defensive positions in the western desert and elsewhere using nothing more than bayonets.
And that is the point that I don´t get. If you carry a fighting knife that only purpose is to cut your enemys throat or stab it in his kidney why don´t leave the fighting knife at home and take your M9 or M7 with you that can do the same job. You also can use it to cut, stab or to fix on you rifle(just in case everybody is out of ammo) or like senojekips said, just to striking fear in your enemy in a RIOT (I think thats a very good point!!! You can´t do this with just a knife that you hold in your hand!!!). That is one thing that you can do more with a bayonet than just with a knife.Still craving for a full sized knife? Get yourself a good fighting knife.