They are a lot of us lurkers out there. We watch a long list of blogs and websites but hardly ever make comments. Well, occasionally I do make a comment. Why are comments important? If you disagree with the blog post, you can express your own opinion. By the way, I have been corrected by commentators more times than I can count. Sometimes the comments are questions that lead to additional blog posts. Sometime comments help me focus on the problem that needs clarification. A lot of times commentators tell stuff I have no clue about.
Lurkers aren't bad. They are very good actually. It shows you care enough to learn from blogs. But at the same time, don't hesitate to be a commentator at appropriate times. In some cases you can subscribe to not only the posts, but also the comments. Think about doing that.