It always cracks me up when I hear guys who have never been an RB with 25+ carries a game say kneepads don't help.
Contusions and bruises hurt my friends...they ache...they make it take longer for you to get warm and loose, and they accumulate in number and intensity as the season goes on. You simply aren't as fluid when beat up, no matter how "tough" you are.
Not having knee pads is MUCH more about fashion than effectiveness.