The J-31 does have potential use when paired with an AEW&C platform. The J-31 is cheaper and somewhat stealthier than the J-20 (the canards don't make the J-20 unstealthy, but they create trade-offs in the RCS, and the J-31 is just plain smaller). The tactic the US will use will be paired IRST / AEW&C radar, i.e, the AEW&C is used to detect opponents, and the IRST on the fighters are used to track opponents, since the AEW&C is bad for that due to low wavelength and the IRST, due to its narrow field of view, is better used to track than to detect. The idea is that if you're fighting stealth fighters without an AEW&C escort, the AEW&C detects them at long ranges, and the fighter, deployed in a dispersed net, tracks and kills them long before they near the AEW&C.
The J-31, combined with Chinese AEW&C platforms, can perform a similar tactic. J-20s could also be used, but they're going to be more expensive and are better used knocking out enemy soft targets than cruising at low speeds waiting for the AEW&C to pick up opponents.
So the idea would be that the J-20 acts to wipe out enemy AEW&C and tankers, while the J-31 + AEW&C act to wipe out enemy stealth aircraft. This isn't to claim that the J-20, in a mature configuration would be unmaneuverable, or that the J-31 would be more maneuverable than the J-20; it's just a matter of cost, the J-31 is a medium fighter, the J-20 is a heavy fighter, and the J-20 will have much greater range and speed than the J-31. You use the cheap asset as the escort and you send the expensive asset out to hunt for AEW&C, because the J-20 mission is high K-D ratio and likely to preserve expensive assets. The J-31 mission, on the other hand, much more can go wrong and a lower K-D ratio could be expected, so you're better off attritioning a cheaper platform.