That's exactly the problem. From what I've seen, the reason people don't want to hire overqualified applicants is because they know those people would be doing exactly what you are advising: coasting in the job for 3 months while waiting for something better to come along. There's nothing wrong with that from the point of view of the person, but what about the company? They spend time and money training someone, only to have the bail at the first sign of greener pastures? Where's the return on investment there? Employers tend to prefer to hire someone who at least seems like they might stick around long enough for them to get their money's worth.