Have not monetized a blog yet personally... but I've worked on company blogs for others. All the reasons stated in the post above apply to one blogger or another.
I started my personal blog just because I enjoy writing and sharing opinions. It's an extension of having been in political discussions in forums waaay more than time shouldve permitted over the years. Long as I'm at it I suppose monetizing 'em for a little profit is appropriate. My wife's grown partial to having a roof overhead, so if I spend the time it should at least pay me back.