An argument against this has been that, regardless of the phone's status -- locked or unlocked -- you are bound by the terms of the contract you entered. Specifically, you are still paying the monthly fee and so paying off the subsidy on the phone (one can argue, although a true, cost-based accounting argument might have you paying off that subsidy in a smaller subset of the initial months).
What unlocking then provides, is the ability to escape onerous terms and fees for additional service, such as roaming fees when traveling outside of the normal service area.
If, with a GSM model, for example, you can pop in a different SIM under those circumstances, should you be able to?
Given some the the shenanigans that have occurred in this industry, including outsized roaming fees, I tend to be in favor of saying "yes", to this.
Or, put it this way: Competition's good. Even, or especially, within the term of rather long (with respect to technical and economic developments within the telecommunications industry) two year contracts.
If, with a bit of effort and trouble, you can escape absurd roaming fees, new data cap restrictions, et al. Then, perhaps, your current, contracted carrier will feel more incentive to compete with reasonable terms and pricing.