First off most components are "locked", only certain AMD CPUs and the HD 6950 can have additional hardware unlocked. Other components can be overclocked, however if you haven't been paying attention to Sandy Bridge and the Phenom Black Edition processors you can tell that Intel and AMD are trying to commercialize overclocking by herding people to buy higher priced "overclocking edition" hardware. Intel has been especially aggressive with this overclocking commercialization effort.
Overall I'm sure Intel and AMD don't like the idea of overclocking and unlocking because it cannibalizes sale of the higher end components, but at the same time they would face riots by the enthusiast community if they completely killed off overclocking. That is the reason why they are moving to the model of locking down CPU so you can't overclock without buying an unlocked processor.
The benefit of buying a higher end component rather than overclocking and unlocking is that with a higher end component you are guaranteed that extra performance and have a warranty, while there is no guarantee that a overclock or unlock will be successful and any overclock or unlock will void warranties.
Now about the 6950 in particular. The HD 6950 and the HD 6970 are the exact same piece of silicon, and in order to differentiate the two of them AMD uses the video card's BIOS to disable some of the cores and lower the clock speed. So by using the BIOS of a HD 6970 in a 6950 you can convert the latter into the former. What AMD is likely to do soon is to start releasing 6950s that have been physically locked down by fusing off some of the cores to prevent any software level BIOS hack from being able to use the additional cores.