Yes there would be "dirty" ways to send raw data but there is a difference between being able to send TCP data to a socket and USB. The difference is, if you send malformed data to a socket, worst case is the server disconnects you for sending invalid data. You send malformed data to a USB device, and best case, your computer crashes, worst case the device gets ruined. Devices have "control sequences" that can screw things up. For example (fake example) consider a printer. If you tell the printer to "spray ink" before you tell it to move the print head over the paper it is possible it may spread ink all over its internal components. With a scanner, you send a command to move left/right. Well if the scanner head is already all the way to the left and you tell it to move to the left you might burn out the motor. Most companies don't make "safety" features to prevent such things. The reason is, the company makes the device, the company makes the driver, therefore the company doesn't care about "what if a user writes a different driver that isn't as safe as ours?" Because such things would most likely void the warranty anyway. There are many many things that could go wrong. mIRC scripts can't just cause backdoors anymore, they can now destroy devices. Consider a digital camera hooked up to a USB port. I send a control signal to tell the camera to open the appature up, and I make it open it so much that the light damages the sensitive components inside. Or I send codes to a USB hard drive to make it spin out of control until it overheats and burns out.
There is no safe way to communicate with a USB device other than using the device driver, and if you are using a device driver, than you can only communicate with specific devices that you know how to work with the driver. All in all, it would be foolish to add this to mIRC.