Looking up the C172M POH I have to hand (don't have a 172N, but don't imagine there's a signficant difference) I think I've come around to BPF's viewpoint also - although the argument I made originally probably is the reason for the checklist under discussion.
That said, a little story. I was testing an Auster 5, rebuilt after 10 years in a shed, basically the same fuel system as a C172.
I did a 5 minute high power test on L - fine, ditto R, fine. Selected both, ran high power fine. Did taxi tests, launched off to fly a sortie planned to use about 60% the available fuel.
A while later, and about 20 minutes flying time from the airfield, I realised that the valve was drawing from only one side whatever setting I had. A very careful flight back to the field had me on the ground with about 10 minutes of fuel remaining in the tank it was drawing from, and an incipient sense of humour failure.
Which wouldn't have been picked up by this checklist either of course as I'd already exceeded what that included.
G