It's worthy of mourning when one finds out that a friend isn't there for the tough times... that someone you'd consider a comrade-in-arms runs out on you in your time of need... that your teammate ditches you when you need him/her the most.
I know of so-called friends like these, and I have no qualms about dropping them from my circle. I can be heartless (and have been), not ever thinking twice about the dismissal of someone I can no longer, with any sincerity, call my friend. It's not an act of cruelty, but of self-preservation. It kills me when I'm betrayed.
That's my stand. I have a feeling the next year is going to bring about a shift in who my friends really are... who to keep close and of who to let go. Fuck with me, and you're gone. As simple as dusting my hands after a fall.