I agree that these UIs make you more likely to buy the same OEM phone again. I've been using ICS+ iterations of Blur and I absolutely love it! Smart Actions is a big part of my phone now, and the way they've added the quick settings to the HD Models is IMHO much better than cluttering up the notification bar. I also like the quick shortcuts on the unlock screen for phone, text, and camera. Overall, even if Moto doesn't push the latest and greatest hardware (such as 1GB Ram vs the 2GB the S3 had), I was sold on the Maxx HD because of the UI (among other things, like build quality, SD Card, and the amazing battery life).
Am I missing out on some of the TouchWiz features? Sure, but the only features I really liked were S Voice (I use Google Now and Speech-to-Text software), and NFC Sharing (it's a standard for all flagship phones now), but unless we're talking that multi-window feature, there are always 3rd party or alternate versions of these features that you can use without leaving the OEM you are most happy with.
If people love the features TouchWiz has, or the awesome little features that Sense offers, then I can see why that would make them commit to that brand.