What I am asking is how much of a difference does dexterity make in relation to missile weapon use. Is there a significant difference between an archer with 20 dex and one with 21?

There's no a whole lot of difference. If you can get an extra point of dex take it, but if you've got 20 your already good.

I know its good, but what does that single extra point do?

A 20 dex gives you a plus three bonus to missle weapons and a minus four bonus to ac, a 21 dex gives you a plus four bonus to missle weapons and a minus five bonus to ac.