The decision is whether you want to own your own domain (e.g., backpackinglight.com). If you have your own domain you have more control, but there is nothing I do right now that I couldn't do with WP.com. I could set up email accounts and other goodies. I have more storage space.
So why did I opt to go with my own domain and host it on a commercial server?
Because I did not want to be subject to WP policies and possible changes in the future. I am not saying there is anything wrong with WP, but who knows what might happen.
Here is an example. For several years I had my pictures stored on Webshots, organized by albums. I had more pictures than was permitted under the free account and paid $25 a year to be a premium member. Over 9 years my Webshots site had over 1.5 million hits and was the #3 rated site in its category. Friends and family would check the site periodically to see what I was up to. There were no stories, just pictures with captions. Then the site was sold. The new owners decided to change it into a social networking site (eek!) and deleted everyone's albums. Over this 9 year period I had linked a lot of these pictures to posts on a tent trailer forum (pictures have to be linked to an external URL the cannot be uploaded to their server), so now all those posts no longer have pictures but a box that says image no longer available.
I used to post trip reports on BPL, which everyone takes quite a bit of time to do if you are adding pictures and is a nightmare to format text, unless you are Bill Gates. Plus BPL is in turmoil, moderators can delete anything you post (rarely happens unless you are Ken Thompson), the owner doesn't seem to care much about BPL, and I don't want to have my hard work disappear into electron hell as happened with Webshots.
Lastly, the blog turned out to be a good method to communicate with my grown kids. They can check out what I am doing and my son, who backpacks, can check out my trips and thoughts in gear -- mostly gives him an idea of what to ask for on his birthday or Christmas. I guess the kids like the blog idea, my daughter just started her own. Besides, Facebook sucks and I am not going to do the Facebook thing :)