Pro - you are writing. You are probably writing with an audience of at least a few friends or family. You are writing and everyone's standards for this sort of writing are pretty low, because it's BLOGGING, for crying out loud. Teenagers without the ability to spell small pronouns do this. And because the standards are low, you are perhaps writing more often and more frequently and therefore getting better or maybe more comfortable and that's good for word production and creative stimulus. You are writing and the members of your currently small but with the potential for growth audience that love and support may provide visible support and that will encourage you to keep writing.
Con - you are writing words out into a medium that you really cannot control. There are ways to restrict viewers to your blog, but then you are undercutting one of the key aims - to get your writing out there where others can see it. You are writing and have to consider that your high school English teacher may see it (not mine, she died while I was in college and I'm still upset about it), or your mother may see it (and I know she will since she's a subscriber), or your boss (not likely), or a member of the community whom you do not necessarily want to see your true opinions on big life issues (also probably unlikely at this time, but it could happen). You are writing but may upset others with your writing and then have to consider whether this is the way you want to go with your writing endeavors.