We’ve made the inclusion of the unmodified Holo theme family a compatibility requirement for devices running Android 4.0 and forward. If the device has Android Market it will have the Holo themes as they were originally designed.
Google states that it has no desire to restrict companies like HTC from making custom skins. It looks like they have included framework to make it easier to incorporate, however Google is seeking a common theme across devices and it looks like this might be Google's way of forcing ICS on devices in the future.
If companies like HTC want to use Google Market, then they need to either come up with a version of Sense that uses the Halo theme, or lose the Google Market. While this might be as simple as changing a couple of colors, it might also mean a complete rewrite of the code that makes up custom skins like Sense.
Personally, I hope that manufacturers are able to come up with light skins that use the halo theme. There is no need for such a heavy skin on Android phones, and Sense is a great example of a skin becoming too bog for its own good. ASOP ROMs almost always run ~100 MB in size while Sense ROMS are often ~450MB in size. While the end user might enjoy a windshield washer wiping their screen clean when checking the weather, it shouldn't get to the point where skins get so large that they prevent even newer devices from getting updates.