If that's the scenario you're looking at - finished or almost finished PS4 version and unfinished X1 version - then I agree. It makes sense to sign up with Sony and exploit this loophole, if you want to call it that.1. PS4 version is complete.
2. Xbone version is in development.
3. PS4 release is now delayed until the Xbone version is ready thanks to this clause.
Even in that scenario though, the MS policy isn't affecting your behavior, because you were going to release on PS4 first whether MS had the parity clause or not. The point is that I'm not seeing a scenario where anyone has an incentive to alter their development path based on this MS clause.