It's not like it matters if you use good grammar when you write, or whether you check your post for typos or the like, or that your references are correct and your images captured and documented properly. It's just a blog, so it's not like it matters!
Well, of course it matters, silly.
When you hit "publish" on a blog post, it means something. Sending your words out there, those words make a statement about you: who you are, what you do. It matters that you are conscientious about what you say and how you say it.
Think of blogging as a very specific form of self publishing. Even if you self publish, you need to consider professional services such as design and editing, to ensure that your book is free of errors, that the formatting is not sloppy, that your images are legit. On a blog there is a lot more leeway--professional services need not be employed on your posts--but it still matters that you put your best foot forward.
Of course, language on a blog post can be more casual, and you'll find that speaking in your true voice brings authenticity to your posts (that's so very important--that's where connection happens), but you are a writer; proper grammar and usage should not be foreign to you.
Recently, I met with Stephanie Clinton (of Hugs, Kisses & Snot) to discuss blogging. Stephanie recently began to work with me on the Oklahoma Women Bloggers, so this is a pretty frequent topic of discussion for us. We both have a lot to say on the topic, and Stephanie is posting this video on her blog as part of her "Hugs, Kisses and Friends" Chat show
She produced a little video clip of our discussion (placed just below for your viewing pleasure).