Why would anyone not want to be a freeman? Well, largely because being a freeman was not all it was cracked up to be. You had no guarantees in life at all, let alone job security. You more than likely could not afford the dirt you walked upon, so good luck buying enough land to build a large shack, let alone a house, for your family. Sure, you could go anywhere you liked, but so could a clansworn serf, so it was not like that was a special privilege. Plus the clansworn always had a home to go back to and knew it. Sure, you could find a trade and ply it to earn enough to set yourself up. Good luck doing that when you had to rent out the property you use to do your work in the first place while having to worry about the guilds and their dues breathing down your neck.