No, I am not saying they have to look almost brand new to be collectible. Some can just be harder to find makers, and you take what condition you find, if or until, you can update it.
Folks reasoning for collecting can vary. If someone is collecting just to have one of each (whatever kind of knife or bayonet) then condition may not be important to that person. If someone is collecting as an investment. And down the road you may intend to sell them instead of willing them to someone. Condition then is a very large factor.
Just because someone pays $50 for a knife and another pays $125, doesn't necessarily mean the $125 knife is in better condition or worth more. I know when I started out I was paying more then something was worth. Sometimes from impluse buying, sometimes it was inexperience. Sometimes from wanting something from my past.
Bottom line is we all collect for our own personal reasons. If we are happy with our own collections, that is what counts.