The short answer is yes.
The longer answer is maybe. Some issues:
1. PLC products are designed with 20 year lifecycles in mind, whereas PC technologies run through fads every few years. The now ubiquitous USB port has only been around for a few years, and who knows what will replace it in another 2-3years?
2. Plug and Play is nice, but the firmware in the PLC to support it now has to track the Windows OS standards, with all the potential version and compatitbility issues this creates. Bear in mind that PLC's get shut away in electrical cabinets for YEARS without anyone looking at them. What happens if in say 8 years time someone wants to attach to a PLC USB port, and it doesn't work because that antique old USB 2.0 embedded in it only works under " a totally obsolete and unsupported Windows XP that nobody has any more"? Like how many of us have a dedicated DOS machine just to cope with the odd PLC that can only be accessed with a DOS driver?
3. And such a port can only be used for programming, because in order to make USB PnP work, there is a lot of OS software and CPU power required to make it work. I cannot imagine that the PLC guys are quite ready to embed all that functionality into say a PanelView just yet. It's not impossible, after all the PanelViewPlus is running of WindowsCE, so we are not too far removed from achieving this.
It's not a silly idea. These days we have become so accustomed to plugging stuff into the back of our PC and it all just works, that we forget the rather large amount of software and firmware under the bonnet that is making it all go. The question is then... could a rapidly evolving and mutating USB PnP technology be transfered into the automation environment, without loosing the long-term robustness we require?