Why? Because Steve Jobs says so? Again, there's utility to having something ultra-light. There's very little practical advantage to having something ultra-thin. If anything, I think it's safe to say that the latest tablets have reached a point of diminishing returns on thinness.
They aren't close to reaching diminishing returns on thinness yet. When I can put a tablet PC in folder alongside paper, each about equal thickness, then we talk about it being too thin.
If you're trying to hold the device with one hand, anchored with your thumb while trying to keep it on the bezel, then it's way more comfortable if your thumb doesn't have to be butt up against your index finger. The human thumb is designed to be opposable. When you grab something thick (at least 1"), you can see your thumb turn over to oppose your fingers. Try to do the same thing with something really thin. Your thumb doesn't turn over, and you end up simply bracing the object against the side of your thumb. Your thumb has little leverage this way, and it's uncomfortable as hell.
Honestly, we must have different anatomy. I just picked up a sheet of paper and my thumb turned over. Furthermore the way you hold the tablet with one hand is with four fingers in the back, not just the index finger. Given the fact that of the 30+ million iPads used I've heard this "it's too thin to hold" argument surprisingly infrequently leads me to believe its less of an issue for the general population than you might conclude.
I can guarantee that Microsoft won't ship Touchpad size/weight tablets.
Easy to guarantee because MS doesn't ship tablets. But their partners do. And I hope they don't ship things that look like the Touchpad. Again, they really should look at the iPad2 and say, "That's the ugliest device that we'll ever ship". The iPad2 should be the baseline for the lowend device. Unfortunately these little previews that show off features like "BluRay support" or "removable batteries" make me fear that their partners still don't get it.