Only holding a stretch for 10-15 seconds likely will make you more tight. When you begin to stretch a muscle you activate the stretch reflex which then contracts the muscle to prevent the muscle from lengthening. It's a defensive mechanism. If you hold the stretch for 30-60 seconds (or repeat this 10-15 second stretch over and over again) you'll eventually overcome this stretch reflex by activating the GTO (golgi tendon organ)and the muscle will relax. This is the whole purpose behind static stretching. The problem with static stretching is that it is entirely unfunctional.
Required range of motion is sport dependant. A runner doesn't "require" much in terms of range of motion, but this is a double edged sword as the extended exercise through a limited range of motion leads to muscular imbalances. You want to increase range of motion? Try this test. Do nothing but easy easy running for two weeks. Then do a complete flexibility test. Then, drop the volume in half and include sprints every day. Test again. My guess is that your scores will be better. Why? Sprinting requires and imparts are greater range of motion in the working muscles.
These more explosive and dynamic means of increasing range of motion activate the GTO (golgi tendon organ) which works to relax the muscle so the tension placed on it (from explosive/dynamic movement) does not cause tearing.
PNF stretching is another means of dynamically stretching the muscle, but again it's not entirely functional either. Drills such as high knees, butt kicks, etc are great ways of increasing functional range of motion. If you develop various issues related to range of motion that cause injury then you'd want to try some PNF to solve the problem or better yet some ART Therapy or foam rolling or other means of getting at the microtrauma scar tissue tha cause most running injuries. So, most of the time the issue isn't exactly a range of motion problem but likely a scar tissue problem. So in a way stretching only lengthens muscle around the scar tissue without exactly addressing the problem.
Anyway, I'm rambling....
Alan