Yes you should. That's what I always tell people who ask me. Most of hese people are evangelists, people applying for evangelism roles, speakers and trainers.
But I don't know what to write about. Everything seems to be written already. That's the number one concern I hear most of the time from these people who are thinking of starting a blog.
So here are the tips I give these starters:
1. Don't blog for the bloggers
Don't be paralyzed by believing you need to do something similar to, yet different from your favorite blogs. Your audience isn't the bloggers. It's the great unwashed. Those who are looking for information through search engines and not yet in their dozens of RSS feeds. You will know who your audience is after blogging a while.
2. You have an interesting live
If you are a techie then you are experiencing a couple of things per day that are worth sharing. If not, you must be on a holiday. So if you are wondering what you can write about, you are trying too hard. Remember, a lot of the value is in the learning process. Once you're an expert, it's tough to step down the ladder; teachers will recognize this. If you blog about what you are discovering/learning, it will be interesting to those one step behind you in the process.
3. It doesn't matter whether you're a generalist or a specialist
Just blog. You will find out what type you are later. As mentioned in point 1, you will also find your audience later.
4. Get Rhythm
Frequency matters. Twice a week is a minimum and more is better but don't stress yourself out.
5. Get a tool
Still doubting? Stop it, you could have gotten your blog already and started your first post.