There is a big flaw in your reasoning: a great many of those seeking things to download dont use common sense and arent very puter literate, and many havent much of a clue the ways they can be subject to virii/trojans/backdoors/exploits or what those things are capable of. They may not even be aware of what "common sense" involves in this case.
For that matter, i wonder how many users, including those those opening their puters to filesharing, have taken steps to protect everything else they have, like documents with tax info, credit card/financial info, social security numbers etc from prying eyes.
It isnt up to me to judge what's legal/illegal or to split hairs by saying that since i can tape a tv show and look at it later (in spite of any fine print that says im not allowed to reproduce it) that its therefore ok to share it with a few hundred strangers online. The bottom line for me is that there is just too much inherent danger in rampant downloading. mIRC was written as a chat client and wasnt intended to be hit and run file sharing central so im not here to help someone find files (most of which are deemed illegal to share by their creators) that put them at risk of exploitation.