DIY LMS or get one pre-made?

I've been wondering about Learning Management Systems (LMS's). How exactly is learning management different from teaching? Yes, I know all of the discussion about online content storage, assessment creation and data tracking. All of that really just begs the question, though. After all, in a typical teaching situation, there is a person who serves as a repository of information (supplemented by a textbook) who gives out assignments and keeps track of how students are performing. An LMS is really just a way to do all of this digitally. 

Since an LMS is a digital "teacher", can't we as teachers just make our own LMS? And are formal LMS's performing any better than ad-hoc systems? I've read quite a bit on both sides of the argument. So far, I'm not wowed by the pre-fab products. A well designed system can provide students with the essential items. First, they'll need review materials (videos, presentations, text) to understand an idea. Next, is a way to work with the material socially or alone so that learners understand ideas rather than memorize facts or phrases. This is typically done by assigning some sort of assignment. Finally, the system also has to have some way to assess students and share that assessment data with the students in a secure and sensible way. Usually, this is in the form of a gradebook. 

I've put these ad-hoc LMS's together using Google tools and a bit of hard work and they've performed pretty well. The biggest challenge (besides creating the actual learning material) was to decide how to organize everything so students could easily find what they wanted to quickly and easily. However, since all LMS's have a very similar design, it wasn't terribly difficult to get it to work about as well as Moodle or Blackboard. The most important thing I did was to keep the system consistent with what I did in the classroom. I think if a teacher does that, any LMS is as good as the next.