Honest question: Revenue streams of the host company are an important consideration in choosing a blog platform for you?
With that said, Tumblr has a very bright future ahead of them.
Answer to rhetorical question: because we can't be bothered, so we'd rather just pick a provider who is more likely to stick around than not.
LiveJournal & Six Apart have longstanding revenue models but aren't hosting users like the President of the United States or large corporate media entities.
It's a reasonable enough consideration for folks like us in this venue, the parent comment just jumped out at me as an interesting reason not to give the platform a whirl.