... valid reasons to want IPv6 connectivity: it might be faster or more stable than IPv4, due to load or due to avoiding NAT.
Reasons to want IPv6 server visibility: testing! Better service to folks on native IPv6 connections. In the future, I could dedicate an entire IPv6 address to each kite and relay raw TCP/IP streams - even UDP traffic? I could start relaying SMTP!
Reasons to want to disable one or both: to simplify things and eliminate edge cases. KISS: keep it simple, stable.