Avert your eyes, long depressing post incoming.

- Anything involving mIRC being required to be registered is moot as I see it - I'm sure nobody here needs telling that cracks and keygens for mIRC and damn near every program which requires registering are freely available online. Anyone involved in filesharing would very easily be able to find these things. Conversely, people who would have used their unregistered mIRC to transfer legal files would likely find it harder to get one of these things since there's a lower chance that they would know where to get them - or of course they might choose not to use a crack or keygen even if it were available to them.
Main people hurt here: Legal users.

- Filesize limitations can very easily be gotten round as I explained earlier. It would probably take 5-15mins to rar and volume a 1gb file at regular compression on the average modern PC. It only has to be done once, then it just gets passed around and stored in that form, each receiver only decompressing it once, no big deal for them. On the other hand a regular user wanting to compress a legal 1gb will probably rar and volume it in the same time, then delete the volumes once it's finished sending to save space and because they don't expect to send it again soon. Then maybe a week later they find they want to send it again, another 15mins spent compressing. So the legal user ends up wasting the 15mins evading the limitation every time he sends the file, whereas the illegal file was only compressed once, and then extracted once for each user who downloaded it.
Main people hurt here: Legal users.

- Filetype limitations: /rename, need I say more?

Then of course there's the question of whether mIRC preventing or hindering filesharing is actually helpful to anyone. If mIRC removed DCC send will warez on IRC stop? No. Will it be slowed down? For maybe 5 mins while they're downloading new clients and scripts.
So now lets say everyone is using different clients, webscripts, old versions, and mIRC scripts to use DCC:
- The same amount of files are being sent so the copyright holders aren't better off and the IRC servers and ISPs aren't better off from a bandwidth standpoint.
- mIRC in and of itself wasn't being harmed by the fact that it was being used instead of some other client - Khaled isn't better off, possibly worse off in the rare case where a warezmonkey would have actually registered.
- The questions on this forum, on IRC, and in emails to Khaled won't go down. If anything they'll go up from people asking where DCC has gone, or asking/demanding that it be put back in - nobody is better off here either.

Compare this to what happens if an IRC server with competent staff choose to stop filesharing channels. Any significant gathering of filesharers in a channel can be dispersed/removed within an hour or two. The filesharers might have made arrangements on where to go next, but once they've been 'chased out of town' half a dozen times then I'm sure the majority will get tired of it. They'll either go to servers which don't care about filesharing or they'll use a different protocol. Not exactly a solution to the problem of filesharing in general but at least they're off decent IRC servers which have chosen to be about chatting - WITHOUT stopping legitimate users from sending the occasional file. OK so the server then becomes at risk from revenge attacks, but quite frankly attacks are a part of IRC and usually the strength of the attack will be proportionate to the size of the server and it's userbase, so it has a good chance of surviving the attacks. It also requires that the server has good staff, but then again a server without good staff has little chance of surviving for long anyway. Well it's hardly ideal but it seems to me to be a damn sight more disruptive to filesharing than anything that any IRC client could come up with.

Sorry to be the doomsayer but this is just the way I see the situation. As for anything that doesn't make sense I put forth the excuse that it's now 1am here.

Spelling mistakes, grammatical errors, and stupid comments are intentional.