Don't get me wrong. I think that a writer absolutely should strive for characters having a distinct voice (even if I give less importance to it than you do and obviously disagree on methods to achieve it), but I have a tendency to make such statements about what a writer "should" do a lot. You, on the other hand, seem to avoid ever telling a writer what they "should" do. It seems to me that you even chastise posters on occasion for taking a strong stance on what writers "should" do out of general principle. (Am I wrong on this? It's my honest impression.)
It certainly seems like, on the issue of character voice, you're doing what you normally stand against.
I'm just pointing out what I like as a reader, for the most part. I definitely notice generic writing, or characters that all have a generic voice. Sometimes it is done very well, so that's great. But a lot of the best books I've read, both old and current, completely fly in the face of this kind of advice, or any of the other rules of writing that people seem to think are necessary to make a better book. Those books would be ruined entirely if their authors had heeded this kind of advice, so out of my own personal preference to preserve good books of all kind, representing a wide diversity of writing styles and approaches to fiction, I discourage anyone from thinking these rules are absolutes, or that they'll necessarily make your work better. I know from personal experience as a reader that this is not the case.
They'll make it different, and if you're going for that certain type of style, then fine. But if you're not going for that, then you should feel free to ignore this sort of thing, and new writers particularly shouldn't be made to think that they have to follow these pronouncements if they want to write a good book.