I play more than 10 characters, in fact I play more than 20 or 30 characters. Just because it’s different from what you do doesn’t mean is not a legitimate way of playing.
Why would anyone run the same instances over and over again, I get bored of dungeons by the time I’m geared for heroics. Does that mean M+ makes no sense? No, it’s just a different way of playing, that is not for me.
Why put arbitrary restrictions on how many characters are a good enough number to have? The minimum for me is one blood elf of every class and then a hunter of every race. Then there are a few characters I levelled over the years. My first ever character was a human priest that i then race changed to draenei. Should I delete my very first toon because she doesn’t fit the way I play the game now. The first monk I ever played was an Alliance pandaren. Also don’t want to delete her, and so on.
I get attached to my characters I don’t want to see them disappear. Even the ones that I don’t play right now. I see no harm in increasing the number of characters, specially since they keep increasing the number of races and classes we have.