Flexibility can help you as a cyclist, but not in the way that you might think. Becoming a yoga expert won't make it so you can ride with a flat back 24/7 and be super aero. There used to be a body of stretching literature on some bodybuilding/weightlifting messageboards because it was believed that elongating muscle fascia helped with myriad things important to those types of athletes.
It can help if you work a desk job or spend a lot of time sitting around, you can create reasonable power in a position that is limited by poor mobility, or if you have an existing imbalance due to previous injuries. I've dealt with all of these as have many of the customers that frequent my fitter. Sitting down all day and walking short distance can create imbalances of its own often within the hips and/or glutes. Stretching won't completely alleviate it, but it can help you preserve what you have from getting worse. You can become slightly more flexible than you are now, but there is a genetic limit to fiber lengths and its not as if you can somehow stretch and make the fascia longer and more effective over a greater range of motion. Most pros stretch because cycling isn't that healthy of a day job to have- you basically sit down and do a very narrow motion for hours on end, then sit/lay down to recover. Its easy for them to develop slight imbalances due to how much work they do in one static position.
Pros that work on flexibility for time trialing typically do it because they have poor lower back/hip flexibility and can't maintain a more aero (tested in a tunnel) position because of it. Andy Schleck is a great example. He pretty much never uses his drops and rides super stretched out with a severely pronated back. It might look long and low on paper, but its not very balanced in real life. As we all know TT positions can result in a more closed hip angle given the UCI rules that the riders are bound by. If their hip mobility is limited by a few degrees then some stretching might be able to help them more comfortably train in and adapt to a position, however, there are absolute limits to this.
Unfortunately most flexibility issues have been determined by the time one is an adult due to genetics, limb proportions, lifestyle, activity, and injury. With that said I stretch regularly because I have pulled my groin weightlifting, have had lower back problems due to a core imbalance from bad posture in an office chair, and I tend to be slightly more flexible on my left side due to the way I sit for 8 hours a day at work. It won't make me super flexible or ride faster, but it helps me ride more comfortably. I've read that Tom Boonen had to do lots of stretching because of his lower back problems, which he actually accumulated from riding with too aggressive of a position, and that Cancellara does none because he is able to obtain a good position without it.
As for Yoga think of it objectively. A lot of inactive and unfit people go in and work on doing one thing for hours on end. They hit a limit and then figure out creative ways to link stretches. There's a few benefits to what they're doing, but none of them lead to a more effective athlete if there wasn't something off with them before. The positions they obtain would be catastrophic if they reached them during intense exercise.
With all that said I will always continue to stretch until my lifestyle changes and I'm a bit more active when I'm off the bike. I had to get rid of years of muscular imbalances from injury and weightlifting and stretching did help cure my hamstring imbalances, by terrible left ankle flexibility (from a skateboard injury) and a hip imbalance that was causing problems on the bike. My power isn't tied to this, but my experience on the bike is better and I haven't had a problem re-emerge since. My back isn't super flat, but I'm actually very aero with it rounded kinda of like Cadel Evans in a TT. This has been genetically predetermined and I will never overcome this so I just have to make sure that I ward off any problems that might emerge from not taking precautions.
No articles that I can contribute, just logical reasoning. People that want to be more aero by getting lower are on quite the wrong track.
Don't take me too seriously. The only person that doesn't hate
Froome.GramzFailed Custom Bike