I noticed some blogs don't have any dates at all? Is this a good idea? I plan to post daily on my blog and I don't mind having the date showing at all, but wondering what is the purpose of not showing the date?
Not showing the date would be for those who feel their content isn't dated, or they don't plan on doing frequent posts, deflating the value of the information if it looks old. If you are going to post daily, or even weekly, having dates will help from an SEO perspective and to your visitors because it will identify the content as new.
eniram's point is good. If you write on your blog daily, you should be posting the dates. Else, don't.
I don't agree that only those posting daily on blogs should include dates. Totally depends on the kind of blog you're running. I personally think dates should at least be on the index page, and then maybe not on the archive pages, but it really depends on your niche.
Also, some people arent familiar with the blog software and arent sure how to set the date appearance. that may be another reason why some display it and some dont.
Like others have noted: it depends on the purpose of the blog. If your blog is about something time sensitive, like current events, then definately do post dates. I think it's up to you as the blog author.