I put my license on a USB stick and I can install on as many computers as I want. I can work on my PC and if I need to switch to my Mac I move the USB stick from the PC to the Mac.
However unlikely it is, I can go to any studio in the world install all the Waves bundles I own and plug in my USB stick and I'm off and running.
For me that is a close to multiple-seat licenses as I need to get without paying for it.
I've always felt that I got my money's worth from WUP for all the new plugs I've gotten every year in Mercury. My only gripe is that new Abbey Road plugs come at cost (usually quite small though). I'm perfectly happy to pay my WUP.