I think that the older you are and the more mileage
you run, the more important stretching becomes. The
primary benefit of stretching is simply to preserve
the flexibility you have - not to prevent injury,
soreness, or magically make you faster or stronger.
Most of the downsides of stretching result from either
doing it too agressively or at the wrong time (i.e. when
not warmed up - always stretch after running, not before).
If you are young and spry and doing plenty of speed work
(which will tend to work your legs through a larger
range of motion), I don't think it is as important.
On the other hand, it's a habit that will serve you
well over the long term. I always do ten minutes of
simple stretches after each run whether I need it or
not - it's such a habit at this point that I feel funny
if I don't. If you are tight for time, just stretch
your calves, hamstrings, and quads (30 secs on each
will take 3 minutes).