I have a rotatable 22" IPS panel and cleartype doesn't work rotated 90 degrees. It makes text look blurry and multi-coloured.
Since tablets get rotated all the time I can see why Win8 doesn't use it the same way as Win7 does.

Long Zheng wrote about this a while ago.
http://www.istartedsomething.com/20120303/cleartype-takes-a-back-seat-for-windows-8-metro/