Here's a study that could be said to justify the lower protein eater amongst us. This is an old study (around 90 years ago) & is unlikely to ever be repeated due to ethics & similar issues. Basically 2 people were put on a diet where the only protein came from potatoes for 6 months! They ate a little fat & a few other bite like sugar & the odd bit of fruit, but virtually all the protein came only from potato.
The result was that they stayed in positive nitrogen balance even after 6 months on potatoes as their only protein source. I know Bill Pearl rates potato as a very good protein source (see his book " Keys to the Inner Universe"), but these were 2 active people who were getting adequate protein using JUST potatoes (NOT potato protein isolate or anything, but whole potatoes).
Now before we get a lot of feedback I'm not saying that eating potato alone is optimal, nor that you'd get the best strength or physique eating only potato, just that it appears that potato alone is enough to sustain someone, even a person involved in recreational sports (as one of the people studied was), in terms of protein & for a long time as well (at least 6 months with zero negative effect).
I found it interesting particularly as I was just eating potato at the time (in fact I might go & eat some more now & put myself back into a positive nitrogen balance
Anyway you can read the study for yourself here.
One really interesting fact was they also didn't get bored eating potatoes..even after 6 months of potato, every meal, every day! I think I might need a change after that long on one food (& I like potato!)