It's important in both a realistic and dramatic sense. Obviously,
every enhancement only highlights some other gap between the sim and
real life flight. I remember when I thought that all I needed was a
good joystick; then it became a joystick better than the CH Flite
Stick, then a throttle, then pedals.

OTOH, in their own right, pedals are indispensable. Besides the
reasons listed by Randy Wentzel, pedals are great because they involve
your feet. The problem with sims is that they only involve your hands,
unlike real airplanes in which your entire bod is sitting inside of.
Sims are already hamstrung by the fact that you're stuck looking at a
single 2d screen, unlike the all-enveloping world of real-life. Pedals
work against that because they get your feet off the ground. Whether
they assist in simming for real-life training is something I can't
answer. But they're definately a great part of flight simming for fun.

My set-up is a WinXP system using second-hand CH Gameport Pedals. Even
though the manual requires a 2-port gamecard, and warns that it won't
work on win XP, mine work okay on my single-port XP system.