If someone would run a script without having any idea what it contained would they be any less likely to stop a script when a warning came up saying that it was trying to make use of COM objects? I doubt it. The scripter could just put in the readme file that if a warning pops up the user must click yes for the script to work. People who will run things without checking them will almost inevitably do whatever they're told by strangers. An exe file is different because it is by nature closed-source, and anti-virus programs are made to look for viruses in them. The number of mIRC script related false positives and succesful mIRC script trojans just goes to show how inadequate AV software is when it comes to mIRC.
Quick list of the commands available in mIRC scripting that could be used for backdoors: /sock*, /dll, /run, $dll, /bwrite, /bread, $com, /com*, on TEXT, on NOTICE, /remove, /rename, $read, /write*, $*code, /dcc*, /savebuf, /loadbuf, /play, $cb.
And those are just things I came up with that I would consider 'dangerous'. To remove creation of unwitting spam bots and the like you're gonna have to remove /while, /timer and /goto aswell. Hell, you'd have to remove all methods of output entirely.
My point being that if someone doesn't have the know-how and doesn't know anyone trustworthy who does then they probably should miss out on certain scripts.