If we assume, for the sake of argument, that Microsoft's new versioning switch is the way forward, then we can debate what the default value should be.
Either it defaults to following the standards as closely as possible with a switch to emulate IE7 or vice versa. Both approaches have issues.
If standards mode is the default, then authors depending on bugs are likely to have the most work to do to unbreak their pages. Such authors are likely to be those least capable of doing the work, but this could be the impetuous needed to get them to learn something about standards.
If emulation mode is the default, then it is likely that very little will actively break, however authors who understand standards and have made use of them, will have more work to do in order to make IE8 use the features that IE7 doesn't support.
Emulation mode will also mean that we will be stuck with having to use the flag in all new pages (if we want to support IE8) for a very long time.
Give a man a fish and you feed him for a day. Teach a man to fish and you feed him for a lifetime.