Here are my answers for both:

Pi cluster: meshing won't save any additional wires here since there is already a wireless bridge set up (between the black Mikrotik device pictured, and an off-camera Mikrotik wireless router.) I chose to wire the cluster simply to guarantee a reliable connection within the cluster. You can absolutely use a Raspberry Pi over WiFi without an Ethernet cable, it would just be a less reliable connection (jitter, congestion), and then of course it would have to be powered conventionally rather than via PoE.

General 802.11s meshing: If all you want is fewer network cabling in your home specifically between switches and access points, then it could certainly help. However, the best choice network-wise is almost always a proper deployment with a central router and several APs (broadcasting the same SSID) wired back to it.