I think what Z-20 needs is not anti-tank missiles (that's WZ-10's duty), but 7.62 or 12.7mm mini guns for providing suppressing fire.
Everyone loves the look of door gunners and their function and more than a few members over the last few years have come out suggesting it would be useful for suppression etc, to the extent that now every post I see about it is frankly a bit annoying.
But I think people are also a little bit too acclimatized with the look of "US/western" helicopters and door gunners and maybe played a few too many FPS games where the killing power of door gunners, combined, give it an impression like it is somehow an essential system for helicopters in the modern era.
IMO, for the kind of missions that the PLA's transport helicopters will be doing, door gunners are less of a priority than:
1. Having a good, functioning helicopter to begin with
2. Having a defense aids suite (LWR, MAWS, decoy launchers)
3. Difficult weather and night operation capability (PVNS)
4. Difficult terrain operation capability (radar with terrain following functions)
Door gunners are certainly useful if you're needing to suppress the area you're landing at and if you are bold enough to land near the enemy to allow your door gunners to be effective in the first place -- but there are only so many opponents where with sufficiently low capabilities to allow you to do that.
Namely, if you're fighting insurgents or irregulars without any credible MANPADS or AA weapons.
If you're not planning on that, well door guns are more of a "nice to have" than an "essential subsystem".
In the case of this Z-20 w/ KD-10s +/- maybe rocket pods, it obvious they are not intended to merely suppress enemies but to destroy AFVs and/or formations of soft skinned targets (respectively).
If the PLA ever has to fight a COIN or low intensity conflict, then I could see them installing door guns more routinely.
But right now? Nah, they're closer to dead weight.