UTM tags are things which make a URL like this:
http://site.com/page-from-website
look like this:
http://site.com/page-from-website?utm_campaign=website_launch&utm_medium=social&utm_source=facebook
The latter URL can give additional details in Google Analytics.
So, why don’t I use them?
The main reason is that it makes the experience less-than-great for the end-user. I don’t like the way a URL with UTM tags looks like. If I share a URL, I try, as much as possible, to eliminate such tags, most of the time.
Another reason is that I generally don’t hesitate in saying something, no matter the audience I will get for what I have to say. Yes, it matters to me if something I share gets popular, but, still, I need to say a lot of things, no matter if they get popular (or viral) or not. I have a fire within me to share some things, to write some stuff, to do some actions. The UTM tags don’t help that much with this.