Is it ever okay to criticize another blogger?

I'm not sure if I've posted this before or not, but lately I've seen a number of bloggers being critical of other bloggers -- usually as a counterattack (if that's the right word) to a post or something. I'm of the opinion if you don't like someone's site, don't read it. There's really no point in making an issue over something said on another person's personal blog. There's a cattiness about it that I'm not comfortable with. (I hope I'm not joining in by starting a thread about it.) I only bring it up because I've seen some pretty well-respected bloggers doing it. Isn't it generally frowned upon?