Who says that when someone with high skills takes a lower-skill job, that that person cannot keep looking for higher-skilled jobs? Or even that the fact of having the lower-skilled job significantly hinders, or demotivates, the search for a higher-skilled job?

Maybe some study (maybe this study?) says so. But I've seen anecdotal evidence that the lower-skilled holdover job does not significantly impede the search for the longer-term higher-skilled job.

I very much beg to differ. I've been in that boat. When I got out of the military, I came back to the US with no job and no place to live. The employment office got me a job almost immediately (veteran preference), but it involved working in a steel mill, even though I had a bachelors degree in accounting, and was an engineer in the military. I worked the job briefly, just to have a job and bring in some cash, but eventually ended up quitting. I didn't quit because I was too lazy to work; it was just impossible to go looking for jobs and attending interviews when I was covered head-to-toe in protective gear, working 10 hours a day with virtually no breaks. Fortunately, I was able to get back on unemployment using the justification that the job was completely out of my skillset and education. It was about three months before I got an appropriate job and went back to work. Now, I'm an engineer, working on my masters degree.

Absolutely. Having a full-time job dramatically reduces the time, energy, and motivation for finding a better job. Sure, it's still possible, but it's a lot more difficult.

I would think that taking a lower-skilled job would hinder the job seeker from finding a more appropriate position in her field. If you have a female accountant, for instance, who had to work at Subway for a while, wouldn't potential employers sneer and say, "So what's wrong with her? Why couldn't she get a job in her field?" I am not insulting people who have to take low skilled jobs in these circumstances, because I am graduating soon and it wouldn't surprise me if I ended up behind the window at KFC, but I am just questioning your argument.

And what happens to the poor fellow who's qualified to work at Subway, but can't get a job there because there only hiring MBAs?