Tom Kurz talks about this (the relationship between stretching and strength) in "Stretching Scientifically". Generally you weaken the muscle when you stretch it so its important to combine stretching with strength training. Static stretching is pretty mild though.

also re: Yoga ... there's lots of Yoga practices that combine strength with flexibility... Yoga tends to pop up as something that has been proven to be beneficial for back pain while pretty much everything else is as good as placebo (including surgical intervention).

People that sit all day tend to have shorter hamstrings (because the muscle is contracted all day) so I think stretching is important. An imbalance in leg muscles can tilt your pelvis and propagate up to your back...