It seems to me that if the sub pixels are positioned left-to-right RBG then ClearType can triple the horizontal resolution. When they are positioned vertically, it could triple the vertical resolution. This being true for black text on white or white text on black.
The font renderer should have enough know-how to determine that A) I have black text on a white background and B) that my subpixels are orientated either vertically or horizontally, and finally produce the best possible output. Anything less seems lazy.
The new iPad with it's 260 dpi screen will have been out months, possibly longer, before Windows8 tablets come to market. Since all these tablet apps have to be rewritten to begin with, why not get the software working well now, before we have to deal with a poor reading experience on shoddy 130dpi tablets. And we all know they screens will be low-rez.
Big text might work on a far field device like a TV. But on a near feel device, like a tablet, it's not going to stack-up well against the competition.