While the line of text you used may work fine on the IRC server you tested it on, there is no guarantee that it will work on a different IRC server running on a different ircd software and/or configured differently. As far as I know, there is no way for mIRC to reliably determine what the character limit is on a server it is connected to, so it makes sense to use a fairly conservative value.
As for the issue with actions, I'd agree that it would be more desirable if each line was encoded properly as an action. Presumably this would also be an issue with CTCPs, although in that case, I would probably suggest that the line should simply be truncated and not split into multiple lines.
I'm actually surprised that mIRC splits lines that are sent via /raw. I would not have expected that to happen, but sure enough it does. I'm curious if this was intentional behavior. It would be nice if a script could use /raw to bypass the character limit if necessary.
