Using M gives you a bit more creative latitude. Period. It doesn't make you a better photographer, but it does let you have better input into your final image.
If you strap a fast L lens on your camera and use it in green box mode, you will never unleash the full potential of the lens. The camera is going to try to shoot with a shutter speed of at least 1/focal length, a medium aperture between f/5.6 and f/11 and the necessary ISO to accomplish that. By always shooting stopped down, you don't get the advantage of the thin depth of field or creamy background blur from the 85mm f/1.2L that you would get if you dialed in f/1.2 as your aperture in one of the manual or semi-manual settings.
If you step up to P mode, you do get the option of varying the settings a bit and can begin to take advantage of the lens.
By using Tv, Av or M....you put yourself in control of most of the variables. It doesn't make you a better photographer, but it does give you the control to take the image the way you envision it as opposed to just letting the camera make the choices.
I honestly don't know why people insist on talking down to others that don't shoot all manual, all the time. I don't know whether they feel it gives them some measure of superiority or what. Funny part of it is, I've seen some very nice shots from people using the basic modes and I have seen some really, really pathetic shots from people using full M.
Photography is a progressive journey. Typically, as you gain experience, you decide you want to experiment. Eventually, most serious shooters find themselves shooting M and RAW. It doesn't always mean that they are better photographers, it just means they have moved on from the basics.
People should shoot in whatever mode they are most comfortable with. Stepping out of the comfort zone from time to time can be both fun and rewarding....but it doesn't necessarily make you a superior photographer. A perfectly crafted photo of nothing is still just that.....nothing. We urge new shooters in our local photo group to experiment, but nobody is ever criticized for using the mode they are most comfortable with.