Iif someone wants to instal mIRC without the user agreement, he just needs to install mirc, get the files once installed and repack them with an install maker, and as making a troyan with a stealth mirc is quite "kiddy" the people who could do bad things with a commandline/silent mirc isntall have done this already.
on the other hand, having a standard and official commandline/silent mirc is really usefull for instance for clean scripters who want to release theire script with mirc but want also to be compliant with the mirc shareware license which doesn't allow repacking/disassembling of the install. a thing like this could be great for a script :
making a nsis or microsoft which will :
1) unpack mirc official setup and script files
2) run the mirc setup with a command-line like "setup.exe -auto -path:c:\scriptdir\ -runmirc -firstruncmd:load -rs scriptsetup.mrc "
this would be clean, user friendy and license compliant
PS : IMHO nsis install are lighter, cleaner, faster than msi install