This is one of John Gruber's better insights as of late. Samsung is making the right move here: sometimes, UI consistency trumps more "daring" moves. Apple's UI consistency is quite good, especially for iOS (albeit somewhat less so for Mac OS X, especially with Lion).
And there's of course the second shoe that TouchWiz is a differentiator for Samsung (as far as they're concerned anyway, users may or may not agree): it's part of the "value" they provide, and it's (tentatively) part of keeping users on Samsung phones: Samsung also has TouchWiz skins not just for Android but for Bada (their in-home OS), for WiMo (Omnia 1 and 2, long dead) and older smart-ish phones.
So from a business perspective, Samsung considers TouchWiz important for phones released under the Samsung brand, they're not going to switch to stock Android UI.
It's worth pointing out that Samsung isn't necessarily keeping compatibility with Gingerbread as they are keeping compatibility with their own TouchWiz version of Gingerbread. For the most part, this is old news. It's reasonable to assume that if Samsung is going to saddle Gingerbread with proprietary UI "improvements", they're going to do the same with ICS. The fact that it lessens the difficulty of switching from one version to the next is an unintentional side effect.
