Flexibility training can be divisive issue. Even I'm divided on it. On the one hand, you can improve your flexibility levels to a degree through appropriate training (stretching) methods. On the other, the gains you can make are somewhat limited, and I'll often argue largely outweighed by the likelihood of injury (up to 80% of dance injuries are sustained during flexibility exercises. That's an awful lot.).
Having spent the better portion of my life in dance training and surrounded by other dancers, I can honestly say I've seen a LOT of time wasted on flexibility training for next to no gains. 85% of your flexibility is down to your skeletal structure, 5% is down to environmental circumstances, leaving only 10% down to muscular elasticity. 10%. All those hours spent trying to push that little bit further, and the best you can hope for is 10%.
In the last couple of years I've been spending time working with people performing at mid-high level in numerous sports, many of whom are reluctant at best to put any time into flexibility training. When no specific training is focussing on improving flexibility levels, that 10% you have control over suddenly looks pretty important. Watching people with a good level of physical fitness and technique unable to lift their leg to hip height without letting their form go seems a waste of all their hard work.
In physical activity - whether dance, gymnastics or martial arts where a higher degree of flexibility is expected, to team sports like football or rugby where a lesser degree is required - a good level of mobility is essential for the body to perform at it's best. If you train strength, speed, technique, agility, endurance, why skip on flexibility when inadequate levels will negatively impact on everything else? Everyone involved in a physical pursuit needs to train their body to be able to perform the functions they demand of it. If you can't get your leg to hip height without your posture going then your mobility is low, and you need to work to improve your flexibility.
Regardless of the discipline you train in, you need to look at the demands placed upon your body and make your body fit to meet them. That may mean stretching and improving flexibility levels to a degree, or it may mean strength training to give you better control over your flexibility. It's a two headed coin, and strength and flexibility should always be trained together, yet all too often it seems to be almost exclusively one or the other.
When it comes to dancers, high levels of flexibility are expected. If you want to be taken seriously and be able to perform choreography correctly you're expected to be able to move fluidly through your body's full range of motion. I understand why so much time is devoted to stretching - it's the drive to ensure you have taken every step possible to allow your body to perform to it's full potential. But stretching is only part of this, and it's importance is massively overemphasised in dance. Once your muscles have reached a certain point of elasticity, they're not going any further. After that you're wasting training time.
I've gone into auditions and been horribly intimidated by dancers warming up and sitting in full over-splits, with their hips at an angle mine could never hope to reach, and panic setting in. Almost everytime, these dancers have been unable to hold their leg above 90 when it's come to centre work. What a waste! What is the point in being able to split over 180 degrees, if you don't have the strength and control to actively hold it there!? If you can sit in oversplits but can't get to full splits in a grande jete you really, really need to refocus your energy and address your flexibility/strength level imbalance.
Flexibility and strength training need to go hand in hand in order to develop a functional range of movement. A lack of active control over joint range of motion (as is often the case in dancers) can predispose them to injury. Dancers would do well to look into replacing flexibility training with strength training. Flexibility is already catered for through the course of a typical dance class, and any dancer who has been training for several years will have reached the maximal degree of flexibility of which their body is capable. They then need to ensure their strength levels catch up and they can make full use of joint range of motion.