Two reasons:
1. Line in and mic in have different properties and the mic in, which works on lower voltages, will produce a heavily distorted result when being fed a line signal.
2. Mic inputs on some/most cheap computer soundcards have some sort of phantom power, designed for use with the mic that came with the computer, and may hurt the machine you plug into it. However, this is not likely as this phantom power is very weak and most equipment have protection against plugs put in the wrong place but why take the chance when the result will be crap anyway?
Cobra - Are ou sure that you don't have a line in on your card? The symbo next to the input usually look something like this
-((>)) (an arrog going into "radio waves")
Cheers
/Ola