It might be handy in that particular situation, but I can't see many mainstream uses where a 2nd card would be necessary.
Maybe if you had 2x 1070's in SLI a 3rd card would be desirable to avoid the microstutter that can be introduced due to only one of the cards taking longer to finish the frame because only one card is used (or at least used to be used) to calculate PhysX. Not sure if later pre-emption/frametime sync developments have alleviated this yet.
Back when I had 2x 9800GTX+'s I used to dedicate 1 of them to PhysX for Cryostasis and Arkham Asylum, but when Fermi came along it was no longer necessary.
The only way to be sure is to try it in a variety of games that support PhysX and the like.
The other thing to bear in mind is that the 970's shaders will be quite a bit slower than an overclocked 1070.