There are many places in the US-CA border which are unguarded or "honor system". In one on a recent documentary, vehicles stopped at an unmanned inspection station and picked up a "white courtesy telephone". And nothing stops you sailing your boat across the Detroit river, indeed, a water event on the St. Clair river (near Port Huron) recently went laughably wrong and the wind took hundreds of Americans in swimsuits (no papers) to the Canadian side. 

I can certainly imagine someone whose living situation or job had you crossing several times a day, the US has places accessible only via Canada, and Canada has recreation areas only accessible to Canadians that are actually in the US. 

If someone is a proper dual citizen of both US and Canada (or any other two nation pairs), does that mean they are free to ignore border controls completely and cross wherever able? Or do they still need to visit a port of entry and do the formalities?

What would this "visit" consist of? Noting that the standard setup at some border crossings is to *literally phone it in*... can they call it in on their Iridium phone halfway across Lake Erie?