One example would be other kernel modules that require it as a dependency. So if you disable loading the ipv6 module, you can no longer load any module that references it, even if you would not have been using ipv6 in that module. The first one that pops into mind is the sctp kernel module. This also used to prevent loading the bonding module but I am not sure if that is still the case. You might try testing that.

Newer kernels have the ipv6 code built into the kernel, so the kernel boot option to disable ipv6 won't do anything regardless. The supported way to disable it is sysctl and the networking scripts.