I think the original idea was centered more around the Pro Channel than a separate vst bin. Vst can be put into the Pro Channel with a wrapper I think ( correct me if I'm wrong) Cakewalk put a lot of thought into a main fx/plug in area in PC. It appears they concentrated mainly on the fine detail of each channel with respect to visual and adjusting those plug-ins leaving vst as secondary. The answer was a collapsible channel area that is great for detail but not so great for a quick glance. Try hand clicking the PC through even 24 channels independently back and forth and you start to get the idea. Add to this the fact that the user needs to examine two independent areas in each channel. The vst bin is simple and elegant.The same could be done with the Pro Channel.
The plug ins in PC are good...some of the best and if you have Pro Channel loaded up with the additional plugs they offer you really don't need much else, but most of us want to use both. I think more people who already have many plug-ins would use Pro Channel more often if they could simply see everything in a bin for those cursory mix views. I agree that it's a workflow killer to constantly need to look at two areas to see what your plugs are doing.
Someone will probably mention that you can open multiple Pro Channels. I like it on smaller mixes but too much screen real estate is taken in larger mixes with multiple PC open.