Some people are comparing IE's support for compatibility-mode-switching to Firefox's "quirks mode" and suggesting that they're very similar in principle. In fact, they are not.
Firefox's "quirks mode" is defined as "standards mode, except for a fixed set of documented 'quirk' behaviours". This means that almost all bug fixes apply to both standards mode and quirks mode. Quirks mode is actually getting more consistent and more standards-compliant all the time! (Of course it can never be 100% standards compliant.) Every new feature we add is available in both modes. The code we need to implement quirks mode behaviours is basically fixed and does not grow significantly over time.
IE's compatibility modes are defined as "whatever IE version X did". They are not supposed to change over time so the code required to implement all of them grows substantially with every IE release. Bugs in them are never fixed (except for crash and security bugs, I guess). I'm not sure if new features will be added to modes other than the very latest mode, but since it would be a lot of extra work and potentially destabilize compatibility, I'm guessing not.
Everyone has their reasons and I don't need to debate which way is better. But the philosophy, implementation and consequences of these approaches are very different.