"Not sure of the logic of how it would be 'managed' though."
ProTools does this fairly simply by allowing the user to 'Shift Click' on the plug-in so it remains open when another plug-in is subsequently selected.
You can shift click on multiple plug-ins to allow as many as you have screen space to display to be open at once.
That certainty would be useful. I find myself relying on visual audio feedback as much as aural..(Probably not good) but after a few 16hr days, jumping between projects it could be helpful to be consistent.
Of course it would be useful – and in my view its ridiculous that it is not included in Vegas & Acid (or any other app that is marketed to professionals). If you have more than one outboard unit patched in, do you make one disappear so that you can see another? Or another analogy would be that if you use a mixer/console, you can only view one channel strip at a time. Its pure silliness to even consider such a scenario. I've been working in studios for over 40 years and have never met or even heard of a professional engineer that didn't need to be simultaneously adjusting more than one outboard unit (and/or console moves) and this includes visual feedback regarding how the adjustments are effecting the audio. But, Sony's software requires us to workaround this basic, fundamental aspect of audio engineering. Perhaps there's a logical reason for it, but I personally fail to grasp what it could possibly be.
I am NOT a protools advocate by any stretch, but this is one feature in their software that really does meet "pro" criteria.