Honestly depends on the implementation. I always thought those old computer on a an add-in boards were very cool (e.g. Fast Track for Apples on a PC, AST Mac286 for PCs in a Mac II, Radius Rocket on a Quadra, etc.)
Lets say you get something like Sergey's 8088 board which is a full computer on a board, tweak it a bit, shrink it and add LPT, Serial, and FDC support and implement it all on the PCIe bus for power and communication. Then you create a VM for it which runs in a window (or full screen I guess). You then route keyboard, and mouse from the host to the add-in board and video from the add-in board to the host. You now have a fully implemented computer that runs separate and independently from the host. It will have all the HW interrupts and real responses of an actual computer so SW should run perfectly. Parallel ports and Serial ports can be installed on the host case using brackets as well 5 1/4" and 3 1/2" drives (with internal cables directly to the add-in board. If you need an ISA bus a wire can come from the back of the card to a box with a backplane and you are good to go.
This would have major cool factor for vintage computer geeks like us. Of course the arguments against it are many:
1. Cost
2. Limited Demand (as has been pointed for an industrial application this may not be ideal but then again this does avoid obsolescence to an extent as you can upgrade the host and keep the add-in card).
3. If your only need is to run SW the emulators are pretty DAMN good and FREE.
But I would buy one. Hell I would even do a kickstarter support for it (as long as it came with the expansion box)
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