No... don't run a gaming card and workstation card in the same box unless using each card for separate tasks and that's a task getting it set up.
If you have a spare GFX card laying around, you should use it ... any suggestion that you won't benefit is without basis in fact. Any GTX card can do PhysX but it carries a performance penalty on the GPU or if assigned to the CPU ... not unloading that task to a GPU that would otherwise be sitting on a shelf is counter productive.
As for only supports old games ? ... old as in Witcher 3, Fallout 4, any Batman, Assassins Creed IV, CoD Ghosts, Metro LL, etc ?
I compare it to going to a car dealership and the dealer offers you the same model with free air conditioning for the same price and you saying no thanks cause you will only use it maybe 45 days in a year. Sure ya won't use it most days ... but when ya stuck in traffic and it's 95F out, are you going to leave it off.
When I do new builds for folks they usually leave behind much of their old hardware ... I always save their old card or pull one off the shelf and install it, set up as a PhysX card. When user picks up box, I will usually run W3, metro or something with PhysX and PhysX off .... no one ever takes it out. Its a very simple thing provided you have two GTX cards. Now if ya asking if I would recommend going out and buying a Phyx card for $100, I'd say no. But not using one that's sitting on a shelf or if ya can grab it for $20, is a worthwhile effort... IIRC, the last one like that the GPU usage on the PhyX card was 8 - 12%. To my eyes, taking that load oiff ya CPU or GPU is worthwhile.
