In the end, stats are not overly important, it just depends on what matters to you. Some people will allow guest posts, but only from sites with certain rankings. Some people, to increase their own rankings will only comment on sites that have certain rankings. I'm not overly concerned with Alexa and PR, but I am definitely happy when I achieve certain rankings, because I feel its a good way to measure how good you are doing against the entire community, and helps you know if you are going in the right direction.