If I ever started my own blog, my primary reason would be for me to direct people to it personally if I am trying to explain to them what I eat and why.
There are a couple reasons for this:
1) People are always asking me questions. I reckon it's not going to stop anytime soon, like... ever. I think it would be cool for me to be able to say... "Well here, check out my website". This would allow me to get my 'best' info to them to give them a complete picture.
2) It's less intimidating. Reluctance would be lowered because it's not on a personal level. Not that I'm against a personal level, but most people need to see something more 'official' than just a conversation. If I already have a blog with all kinds of topics laid out, then whoever I talk to will not feel "pressured" to convert. They will know it's just what I do. You know... like the questions people ask on Paleohacks all the time like... "How do I get my parents involved without pressuring/offending them?" "How can I convince my brother to go Paleo without getting into an argument?" I think having a blog of topics already written takes the edge off.
Funny too... because I've had tremendous success so far with family/friends, even without a blog like I'm talking about. It's like people are drawn to this when I tell them. Almost my whole family is switching over. I didn't expect that but I'm very happy about it.
Adam - I say go for it. As you said, there's a lot of info out there already, but just like starting a journal, it always seems 'too late', but it's never too late.
Put your own spin on it and claim your space on the net.