Allow me to weigh in as the person who was responsible for pulling the previous blog out from the forums. First, you are correct in thinking that fundamentaly, structure wise, they are very similar. This is what allowed the forum/blog project to be one early on. Once you start making more modules more complex, however, you find the differences to be great enough to merit seperation.
Let's look at why I found it to be a good idea for the modules to be seperate, aside from the fact of developing for one took away focus from the other:
1. Forums run a strict Group -> multiple Forums method. If doing a blog using the same fundamental structure, this means you have to go two levels deep using a category system. (ie. no top level blog posts as a post can only be done within a forum and not a group)
2. Trackback functionality. This and all other 'blog' specific functionality is useless in a forum.
3. DotNetNuke Limitations. In the current DotNetNuke Core, it is impossible to uninstall only one of the modules without affecting the other.
4. Look and Feel. Seperation of UI elements, in prior implementation I was stuck with using a UI very similar to the forums. This is not your blog look people are expecting to see from a blog module. People expect to see a category module UI, a calendar UI as an option at least.
5. Performance and complexity. It is a known fact that 'long and skinny' tables perform faster with regards to query results. The forum structure is as close to this as possible. However, forums have lots of information to track, where as the blogs have far less information required for a single post. Consider that most forums will have far more posts than a blog, this allows faster query results without indexing of the table.
6. Comment system on blogs. There is no real way this works in to the forum system. You can attempt to set each comment as the equivalent as a reply, but this requires more overhead than necessary plus you must impersonate an anonymous user in some manner, which defeats one of the fundamental ideas of the current forum implementation.
7. Modular Development. It seems to me a win win situation to seperate such things because if you want to change something in one, you only affect the one module and not the other. This would be true in almost any other combined module effort.
I hope this clears many things up for you. These decisions were made and there is no going back. This wasn't a decision based on what two or three people thought in one discussion, we have spent many hours deliberating on what is best for DotNetNuke and this is what we have determined.