I think you're right.
But there should be a warning letting people know that mIRC may not work on ports under 1024 on a secured computer or linux OS if they lack the privilages to do so.
Considering that mIRC is made only for Windows, there shouldn't be any issues with not having the privliges to run it under Linux. Even if it was emulated under Linux, I assume trying to listen on a port lower than 1024 without privliges would result in an failure to create the socket, and mIRC would already have builtin error handling for that and give an error message or try another port.
Most people won't even change the values there unless they know what they're doing. I'm not saying the default values of 1024 and 5000 should be changed, I'm just saying that we should be allowed to set a value lower than 1024 if we really want. A warning message would be a good idea if you really want to make sure people who don't know what they're doing are notified.