I have 108 people ignored, but mind you, I've been using this feature since 2006. It really adds up after 9 years. heh
I've blocked creeps, spammers, scammers, trolls, and people who I've deemed to be an idiot or overly aggressive in talking to me or my friends. I also rid you from my Gaia life if you PM me about the items I'm selling on the MP ( my store clearly says not to PM me about anything other than removing the tax, and that I'll block you if you don't heed my warning).
I don't like being harassed.
It doesn't take much for me to block a person honestly. Not that I'm easily offended or irritated, it's just that I'm not interesting in the fact that they exist and that they're trying to communicate with me. They're a waste of my time, and once they're ignored, I don't have to worry about them ever again. (Unless they want to play the Mule game)