I think one of the most obvious drawbacks of having blogs replace hand written journals is the open availability of blogs to the general public. Sure, you can make your blog 'private,' but that doesn't fully guarantee that what a person writes in their blog doesn't have the potential to become public. However, I personally prefer blogging over journaling; I type faster than I write. But in the situation of having students blog you have to be more cautious. I think the most helpful thing a teacher can do is to explain to their students, if they do choose to encourage blogging, that what is put on the interenet can never be safely considered private.