Saw one guy in Tractor Sup[ly In Blackman Township Michigan carrying a 5 inch fixed blade and another guy carrying an open buck skinner in Brooklyn MI...??? Legal?

I don't think so. The way I read the law 3" is the limit for knife blades, concealed or otherwise. Some of the language is convoluted regarding hunting knifes "while hunting" if I remember correctly.

It boils down to intent, which boils down to the police officer is making the decision about you and your probable threat to the community. Since I have a CPL I don't carry any knife, not even a small one. I do own a multi-tool and keep it in my truck. I have never measured the blade length, but it's damn close to the limit. I think a LEO would be on thin ice charging someone for carrying a multi-tool, but my opinion would not be the one that matters.

I think the 3" rule mainly applies to those that have unlawful intent.

http://www.legislature.mi.gov/(S(tvc0rq0g1n5fwhxffb5r2niu))/mileg.aspx?page=GetObject&objectname=mcl-750-226

it is legal as long as you don't intend to use it for illicit purposes.  
Note: I am not a lawyer, this in my non professional interpretation of the law.

In any case it's not a good idea, If you have a steak knife sitting in the console of your vehicle it's illegal but if it's in your lunch box it's ok. Not something you won't to get mixed up in.