I very much disagree.
When I was twenty, I joined a new RPing site after two years of avoiding RPing. I joined an RP with six other players quickly, putting up a character sheet for all to admire. It passed with flying colors. All...except one. She didn't like certain aspects of my sheet, and made no secret of her dissatisfaction. The two of us argued constantly for nearly two weeks, before everything went fairly quiet for a month. It was after that month that I got a PM from this girl. I can't recall the exact wording but it was something like 'Hey I like you and want to be your friend'. The two of us have been best friends for nearly ten years now, We have wasted thousands of hours in one another's company at this point. Were she in the same country as me, we probably would be dating. And this wondrous friendship, and the many others that sprouted from it, would have been denied if I had just hit the 'block' button.
Human beings are not monoliths. We do not strictly adhere to one mood or way of interacting with the world, even if most humans have a reasonably predictable pattern of behavior. However there are many things than can alter this pattern, ranging from a bad mood to medical ailments. I occasionally have migraines and act disagreeable. Should that *really* be justification for someone to block someone else? Even in the world without block buttons I grew up in, there were other ways to go about life. You can delete PMs. Ignore topics. Contact Admins. Report bad behavior, or even just turn the computer off.
In my experience, most 'blockings' are due just disagreements of opinion, that could be smoothed over if both sides took the time to talk. Most people you would actually want to avoid are pretty self-evident before the first message, or won't be stopped by a simple block. I just blocked someone for the first time a month ago. A troglodyte whose posted information made it very clear she hated this site and everyone on it. Still, she had some interesting ideas I wanted to try, so I approached her. We were a good month into our adventure, when work became far more demanding (happens at times in the medical field). I had warned this might happen, and that I would reply as soon as possible. I came back a week later to find she had been slandering me in my absence. That's likely the sole reason I would ever block someone, when a show of kindness is betrayed. Even then, I knew the warning signs were there. I knew what I was getting into
I'd like to conclude my disorganized speech with a quote. "You do not silence a man by blocking him, John Snow. You only prove that you fear what he has to say." You probably won't miss the hurtful comment the block button would potentially spare you from. But you might miss the apology that could make all the difference between you two.