A shop that I frequent in Columbus has an interesting spin on this; in addition to their standard house spro they order spro from regional roaster. Typically it is not a SO bean but instead the roasters standard blend. There are a couple of cool things happening with this;
Customers can try something that they are not used to and can see what is going on in the rest of the region.
I see a couple of problems going on here though:
Their baristas are not all professionals with completely developed pallets to determine best grind and dosage for the different spro. So it is not always possible to get a worth while shot to compare. Also the guest espresso is some time only there for a week not allowing each barista to really play around and zero into how their shots should be pouring.
Finally I would love to see something really wild and crazy going on from a guest spro. I think it should really contrast what is going on with the house blend. The option of having a special SO is a really cool idea if a shop is willing to work to perfect each one.