Warm up and preactivity stretching is done to alleviate tightness that might inhibit performance or movement. It is not done to improve ROM (range of motion).
Post exercise stretching can be done to improve ROM or just maintain current ROM. Individuals with good ROM stretch to maintain it and those lacking ROM stretch to improve it. Current ROM can be assess using a goniometer and compared to average or desirable ROM for each specific joint and movement of that joint.
Some activities require greater than normal ROM. Stretching to achieve this ROM can improve performance. But progressing ROM too quickly can be as dangerous as progressing weight training too quickly. And increased ROM often increases the likelihood of injury. This is one of the reasons that shoulder injuries are more likely than elbow injuries.
But the notion that “not stretching is OK” is becoming an “opinion” of many in the fitness industry based on limited research and often lacking scientific methods. If you have good ROM and live a lifestyle that includes movements that are essentially stretching which maintain this ROM, then no you do not necessarily need to stretch in addition to this. But if you find your ROM often makes it difficult or impossible to accomplish movements or tasks that you need to be able to do, then you would be wise to include stretching in your life.
The responses given are right on the mark. One thing to keep in mind is that if the intention of the stretch is to increase ROM about a joint a dynamic warm-up is essential in order to increase the temperature in the soft tissues (fascia) that will respond to the overload of the stretch. It’s also why, in general, it’s most effective if the stretch is held for at least 5-8 seconds.