I agree that the Tablet PC will evolve over the next 5-10 years, especially as voice recognition becomes common place. Right now most TabletPC's are of the convertable type to allow for the use of the keyboard. With near-perfect voice recognition the average user would have no need for a keyboard.
Actually, there have been many studies that indicate that audible input is actually a bad form of input for most use cases. Why? Vocal stress. The human vocal organs aren't designed for continuous and lengthy usage. Dictating your term paper, for instance, would probably take longer than typing it and will certainly cause severe vocal distress.
For reading your text book, being able to say "next page" might be easier/more natural than pressing a button... but not enough so to really matter here, I would think.
I've not used a tablet, but I can think of no reason why they'd be less optimal than reading a physical text book. I'd expect the weight and dimensions would be negligably different. Eye strain might be a concern with the tablet? Battery life certainly would be. Beyond those two, though, what are the issues people see or believe to be there?