LINE?jack is much more amplified than a MIC one... which means that in order to record through the MIC jack you would have to reduce a lot the output voltage (i.e. the level) of your signal... and that's WRONG for the dynamic of your signal.
To explain what I mean by dynamic, let me quote an example...
Let's suppose a?LINE jack voltage range that goes from 0 to 10v... let's suppose a very tiny sound that is at 0.1v level.
Now let's switch to MIC?jack where you've got a voltage range that is 10 times smaller... 0 to 1v... the tiny sound would then have a 0.01v level...
BUT are you sure that the electronic circuitry of your sound card can handle such a small level signal
(sound >< noise)
I don't think so... and that's the reason why there's a LINE jack with specifications that are different?from a MIC jack to avoid what is called DYNAMIC COMPRESSION.
(CDs have a greater dynamic than vynils, and that's the reason why you can differentiate much more easily a sound?from a noise that is coming from the electronic that you're using listening to a CD... and that's the reason why CDs are all over the place now...)