[C Of K]

The biggest reason Midi-Chlorians were included, I think, is probably because George Lucas wanted to include them.

Had they been excluded few (relatively) would have questioned Darth Plagueis' ability to create life, any more than they would Palpatine's ability to shoot lightning from his finger tips. (Anyone who would question it is probably already questioning countless different aspects of the saga. So it would hardly matter.)

We already knew that life creates the force from watching the original trilogy. It would then be conceivable/believable, that certain secrets would also allow the reverse to be true. Of course, even with the addition of Mid-Chlorians, these secrets are practically lost, and probably very difficult to achieve for those who know them.

What the addition of Midi-Chlorians did achieve for George Lucas, was a "simple" way to give the audience more insight into the mysterious force without having to explain a lot of mystical rules. Thereby allowing him to get on with the already long and complicated story at hand.

I can see why he did it, and although I don't really like it, he wisely chose not to focus too much attention on it.