Bloggers are free to choose their own style and monitor their spelling and grammar as they see fit. But for the sake of effective communication, I recommend some proofreading regardless of your sentiments. Writers and editors should be a bit more cautious since this is a reflection of their skill in their chosen profession, but before you go criticizing others, keep this in mind.
1. Blogging is informal. As a less formal writing style, blogging leaves writers with a bit more wiggle room regarding the rules. While you want well-written copy in your blog, you don't want it to read like a college term paper. Communication is key here, so know the rules and break them with care when it seems necessary.
2. There is no editorial staff. When a blogger is publishing time-sensitive material, like a news article, there isn't always time to have a second set of eyes. This is also the case when a ghostwriter may be writing a blog post for a tight deadline. While writers should always proofread their own work, sometimes there just isn't time to have someone else look it over to catch the errors they've missed. Besides, with the exception of blog networks, blogs are often a one-person show in any event, with no editorial staff to review the writing before posting.
Correcting Others' Grammar Errors
While I can't tell other people what to do, my policy for commenting on the grammar of other bloggers is similar to the way I handle other people's errors in conversation. I comment privately or not at all. If I see something that would be potentially embarrassing or may cause misunderstanding, it is better to send an email letting the author know rather than publicly humiliating anyone. This is especially the case when someone is sharing their valuable expertise for free.
The goal, as I said before, is communication. So for the sake of readability, I recommend attempting to use proper grammar and punctuation but keep the tone informal. If you can, type your blog post in your word-processing program. That will not only give you access to the spelling and grammar check feature, but it is also easier to proofread since you can zoom in and out to a text size that's comfortable. Also, if at all possible, have someone else go over it before you post. This can make a big difference.
So how seriously do you consider grammar and punctuation in your blog?