Edit: this DOES raise an important question; will you jump ship for a better job when it comes along? This is rightly what employers would be scared of happening as employing someone is a big investment for them.
Um, why is it a big investment for them the replace an employee? We are talking about roles that doesn't require that high qualification (I'm assuming this, because if someone is significantly over-qualified for a job that requires a high qualified person, then getting a proper job for that over-qualified person or surviving for some months shouldn't be much of a problem anyway). The employer can find a new (low qualified) employee in a short time, and usually educations/courses that are part of the training is bound to a contract (at least in Hungary), so you have to stay for 2-3 years, or pay the price of the training if you are leaving.
According to my experience (with small companies and multinational companies), bigger companies don't care much about fluctuations, as most people in the company are replaceable. Though they might choose an over-qualified person for a low-quality role, because they know they will need a high-qualified person in the future, and they might want to elevate it from their employees instead of hiring stranger.