In my opinion there are more pluses than minuses for a blog in a classroom. One plus would be that the students would have a place to go to for all of their work if they were absent one day. Another plus for this is that the students will not be bombarding the teacher with a million questions about what they missed, all the teacher has to say is go to the class blog. If a teacher teaches 4 classes a day and has 30 students in each class, then if 3 are out from each class this could take precious time away from the teacher getting other work done.
A minus for the class blog with all of the class work on it would be that this is just extra work that the teacher is having to do. Keeping up a blog can be time consuming work. Along with all of the other mountains of paper work and reports that teachers have to do, trying to keep up with even more stuff would be hard. This is especially true for first and maybe second year teachers too.
Another plus for a class blog would be that it would be a good communication device for the students to ask questions about something that they did not understand in class that day. The student could make a post saying hey did anyone understand how to do number so and so on the homework, I'm stuck on the third step. This allows the students to interact with each other and also frees up the teacher, allowing him/her the extra time needed to finish the test/reports/paper work.
A minus for the communication device would be that the teacher would have to monitor this blog very carefully to make sure that it is being used how it is supposed to be. The students might try to cheat and get answers from one another instead of just asking how to do one step. This inturn means more hours of work for the teacher because he/she now has to read all of the posts and comments to make sure that they are appropriate for a classroom.