This post originally appeared as a response to a question posted on the knowledge-sharing website Quora.
Short answer: No.
Any movement that takes joints to and beyond their normal limits of range of motion (ROM) will usually increase flexibility.
Resistance training - including free weights, exercise machines, and resistance bands - is an effective method for developing ROM.
An additional benefit unique to resistance training is that it simultaneously develops strength; think of it as strength-based stretching or, as it is more commonly known, "loaded stretching."
The ability to concurrently improve flexibility and strength presents a practical strategy for older populations, who are at an age when maintaining ROM, strength, and muscle mass is directly correlated to quality of life.
An argument emerging with increasing frequency is that resistance training enhances flexibility equally or even more effectively than stretching.
However, this claim is a misconception because almost all of the research upon which it is based compared resistance training to light-intensity stretching only.
This is an essential caveat because there exists an abundance of data showing that higher-intensity stretching is typically more effective for increasing ROM.
It is also a caveat that many trainers and therapists with a bias towards strength and hypertrophy seem to miss, either because they are unaware of it or purposely ignore it.
Another important condition for successful strength-based stretching is that exercises must constantly challenge the limits of ROM.
As with all things, balance is critical: combining different training approaches in a single workout (e.g., dynamic stretching in the warm-up, loaded stretching in the central part, static stretching in the cool-down) regularly yields the greatest results.
Consistency is also central to the success of any programme. Therefore choose methods and approaches that you will stick with.
If you prefer to stretch for flexibility, then stretch; likewise if you like full-ROM resistance training instead of stretching, then do that.