T-Mobile explains that "software implementation on the G2 stores some components in read-only memory as a security measure to prevent key operating system software from becoming corrupted and rendering the device inoperable." This would be more valid if it were very easy to corrupt your OS. Even if someone botches the rooting of a device, there are plenty of resources to help correct the mistake.
The press release goes on to say: "There is a small subset of highly technical users who may want to modify and re-engineer their devices at the code level, known as "rooting," but a side effect of HTC's security measure is that these modifications are temporary and cannot be saved to permanent memory. As a result, the original code is restored."
Savvy Android owners have enjoyed the openness of the OS, and might resent being prevented from modifying their devices. We can't argue with HTC or T-Mobile at this point, but hopefully this anti-rooting measure doesn't become the norm.
source: T-Mobile via TmoNews