Though I haven't seen such a setup yet in action, or seen the real value of an addon Physx card.
Yeah, I don't think there is much advantage to the physx, other than feeling like I'm getting some use out of the old card. I know darkest of days has some cool effects, and mirrors edge, and maybe some more in the future.
My concern is the fact that I don't think my mobo supports SLI, so I wonder if it supports this feature of using a second card for physx. And will my power supply be able to handle it.