And you save MOUNDS of money using an unlocked device. If you travel abroad alot, you may not need a plan for every month. Also, contracts have minimum voice and data requirements which you may not need sometimes. And smartphones at most carriers need a higher cost data plan and minimum voice plan, though TMo is moving away from that policy.

Unlocked devices have no rules, but branded is stripped and crippled, and usually heavily monitored. For instance, at&t won't allow Slingplayer, P2P, torrents or other apps and features on its network, and has threatened to cancel contracts for violaitons.