The Shift keys get more use than Backspace, and are one of the most used keys on the keyboard outside of the alphabet letters
I find I make the most mistakes in all typing tests when I have to Shift-letters. The problem is that the Shift keys move your hand. Consider the following two words:
Food
Hand
In QWERTY, both of those words are awful to type if you use the standard rule of "use the shift key in the opposite hand of the letter you're pressing". You have to use right-shift for "F", and now your hand is out of position to type the following "o". "Hand" isn't as bad but still illustrates the problem. These are only basic examples. Sure you could use left-shift for "F" but that would then run into the same problem with "Fat".
Using Alt as a shift key solves all problems. You just use your thumbs to shift letters to upper case. This seems like a no brainer for both comfort and speed. This is what some of the ergonomic keyboards do already with all of their thumb keys
I'm curious others think