From an Engineering + Compatibility point of view, we just can't take the hit. There are way too many third party apps that host Trident, and having multiple copies puts those apps out of wack. It's simply impossible to have multiple copies of mshtml.dll on your box and have all of the IEs and all of the third party apps out there behave nicely. Even if we broke a measly 0.1% of our users, we're still talking a really huge number ... we don't have some solutions rolled out because it breaks a bunch of users, and that is not acceptable.

Disclaimer: Even though I work for Microsoft/IE, this isn't an "official" response. It's just my own opinion :)