I hear this position often when people talking about contract-to-perm, and I just don't see it. If you're good at what you do, and people enjoy working with you, why wouldn't you take a contract to perm position? The only reasons I can see are that you are afraid you don't get the job (well, then you wouldn't have been a good fit if they hired you) or you end up not liking the team/company as much (same as if they had hired you.)
So, why wouldn't you? And I don't buy the 'I have a good job' line, because you wouldn't even be entertaining offers if you weren't interested in leaving your current role.