I think one of the biggest advantages is that USB has a faster transfer rate than serial ports. That, and many computers don't have serial ports these days.
I would not give up on your plotter, a communication interface from RS232 to USB is not difficult, and you may just have bad adapters. i would contact the plotter manufacturer to see which one they recommend, if you still want to use it, it is doable.Tim - I have a few of those adapters and they will not work with my plotter - a CNC may be different, I dunno.
I would not buy a complex machine such as a CNC without having a sure fire way that it could be "adapted or modified" to accept USB cables and commands prior to purchase. just my personal view on the topic question.