It is just a way for Lucas to quanitfy FORCE powers within an individual. It is no less mysterious than just having the premise be solely "THE FORCE." It also would explain how Padawans were found in the Old Republic, and how the Jedi Order came to be. It is only a way to guage the strength of the FORCE powers within the individual, and their potential when weilding it. This does not destroy the mystery of THE FORCE, if anything it still remains a mystery as to how or why it is drawn to certain individuals (it is random). And it still does not change the idea that the "FORCE" is in all things (just in different quantities). Midi-chlorians help explain the strength, or lack of FORCE. Also, who is to say that FORCE training does not make one's Midi-Chlorian count increase or decrease, thus FORCE powers increase or decrease??? Also it makes no difference in OT that this is not explained to Luke, because it is not necessary. YODA and OBI-WAN already know the potential LUKE holds, and it is more important for him to understand how to use his gift in a short time period. Why would it be important for him to get caught up in "counting" midi-chlorians or figuring who's got the most Midi-Chlorians garbage???? Plus there are only a few Jedi left at the time of OT. Why would Midi-Chlorians matter at that point, when their is no longer any Jedi Order or young Padawans to single out to train??? Who knows there are still some things we may yet find out about the FORCE and Midi-Chlorians, either way nothing changes in my mind. Maybe if Lucas decided to do 7,8, and 9, then Midi-Chlorians would become important once again, in building a new Jedi Order.