Why disable IPv6? At least there is an easy way of enabling it again, isn't it?
If Tomasz is doing what we were doing before, this is just changing the sysctl setting. Very easy to reverse, which within GCE would only make sense to do if you wanted to use IPv6 within Linux's internal networking stack (e.g. use LXC within the VM and have the different containers communicate over IPv6).
Even now, not all software reacts well when IPv6 is enabled but not available. We encountered real-world cases where this caused a bad user experience, fixed by disabling IPv6. (I'd have to look up the details - this was a while ago.) Of course it makes sense to enable IPv6 wherever that is available, but unfortunately that's not currently supported in the GCE network.