Q:
A: Since those keys are used in combination with other keys, i.e. Cont-alt-p or whatever, you need you those on both sides so that you can get to key you need. This, of course, assumes you're typing with your fingers on the asdf jkl; keys and not picking with one finger. When you need to do a capital A, you use the right shift. When you need a capital K, use left. Same with the cont and alt keys, though they aren't used that much. But it would be really hard to do a cont-alt-p if the only cont-alt keys were on the right side.