The way I see it, it all comes down to two questions each blogger should ask him or herself:
• What are my goals?
• Will blogging help me achieve those goals?
I know it sounds sort of cold and calculating to see blogging as a means to an end, but hear me out. See, I think writers should have an idea what they want to get out of the blogging process before they jump into it. Not only will this help them figure out if the blog is helping them meet their goals, but it will also help them shape the direction of the blog itself. Here are a few classic blogging goals and how they affect the tone and style of a blog.
Blogging for Fun
Most people who have blogs do it because it's fun. After all, if you don't love it, the project can quickly become a chore. If you're doing it for your own enjoyment, things like number of followers or posting schedules don't matter too much. The visual design of the blog is also not a big deal because you're doing it for you, so if you like how it looks then that's what counts. This is a perfectly valid and noble blogging goal, and many bloggers out there do it for this reason alone. In the end, even if your blog goals develop beyond just having fun, remember to take time to enjoy the process.
Blogging for Community
If your goal is to build a community and connect with other people through your blog, things start to shift a little. For starters, you need to make sure your site is user-friendly. A few months ago, I asked my sister (an e-marketing guru) to analyze my blog and suggest some changes to the interface. I never realized it before, but little stupid things--like making sure your font is readable, or making it easy for people to click and follow your blog--can make a huge difference in terms of helping your visitors navigate their way around your site.
In addition to the look of the site, it also becomes important to engage in dialogue with your readers and other bloggers. Blogging for community is kind of like when you find yourself in a serious relationship and suddenly you're thinking in terms of "we" instead of "me." All of a sudden you go from wondering "what do I want to write" to asking "what does my audience want to read?"
Blogging for Promotion
I think when people say "writers should have a blog" this category is what they mean. After all, writers will need to promote their books when they get published, so what better way to do that than with a blog, right? The irony, of course, is that of all the writer blogs I read, very few of them (if any) go over the top to promote the blogger's published work. More often than not, the promotional stuff gets tucked in discreetly--in the sidebar or as the subject of the occasional post--but rarely does it make up more than a fraction of the blog's overall content.
In the end, each writer will probably take an approach that combines all three to varying degrees. It becomes something of a balancing act and depending on what you want to accomplish as a writer, you can give each of these categories more or less weight. Also, remember that it's perfectly OK for your goals to change over time, so don't be afraid to allow yourself the freedom to shift gears with the blog as well.