I think some people do use MCE for communication purposes, but in my experience as an observer, a CDI, and and an educator - I've seen that many of them cannot use it 100% because there are restrictions in when using MCE in a conversational form. It is almost robotic-like. One student I met in college grew up using SEE II. When he joined our college, the college only provided ASL interpreters. It was wrong of them, and I fought for him to get a SEE transliterator. He also got other services such as notetakers, etc. I also took the time to really learn SEE so I could communicate with him. That was a waste of my time because by the time we got to the next topic of our conversation I was exhausted! Then I noticed he really did not use every aspect of the SEE II mechanics (which is what SEE promotes - using all of the mechanics such as adding the "ing" to present participle verbs). Why did he omit some parts? Because it took forever just to get an idea across! So he was using shortcuts.
The problem with the shortcuts is that only he knew what the shortcuts were...thus making communication difficult. I finally persuaded him to join ASL 1 - which he did. He loved it...now he uses ASL instead of SEE. And now we can talk all night long.
His mother told me that she always thought he was not much of a "talker" because he would not converse much. Now he converses nonstop in ASL. She can't get him to shut up!