...in the Phantom Menace* as symbiotic life-forms that help people connect with the force.
It solves the problem I've always had with some people being force sensitive and others not for no particular reason. It felt like election in the worst sense. And the idea that some people could connect with the force because they were 'better' or something, reinforces the idea of smug Jedi.
Midichlorians solve that problem. It's not about the value of the person, there's natural causes that give some people opportunity to do fantastic things.
To me it doesn't destroy the magic of the force. Midichlorians aren't the force and they don't even generate it, they just help people connect with it. Something so mystical manifesting in the biological thats found in all life even adds to the awe factor for me.
And people connecting to the force through the assistant of microscopic things in a symbiotic relationship emphasises the balance and harmony and how power isn't found in the big brash darkside things.
So yeah, for me they fix something that would really bug me otherwise and only increases the Force's mystique.
I believe this is a view that's not shared by many others...
(Though they weren't presented particularly well in the film. Also I was 8 when the Phantom Menace came out so I'd didn't have the experience of having 20 years of previous conceptions overturned which might be flavouring my view a little)
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*The EU was really silly about them. In the film Qui-Gon "Without the midi-chlorians, life could not exist, and we would have no knowledge of the Force. They continually speak to us, telling us the will of the Force. When you learn to quiet your mind, you'll hear them speaking to you." and I assumed that the 'speaking to us' was metaphorical because they connect people with the force and the force is all about the universe wide interconnected harmony etc etc. But in the EU, no, they're sentient and are literally telling you things.