Comment # 19 on bug 1189965 from Michael H. Kelley

The benefit would be that the version information is available directly to
Hyper-V.  There have been one or two times in the past where Hyper-V needed to
do something different based on the version information in the Linux guest ID
because of a bug in certain Linux versions or because of a need to maintain
some other backwards compatibility.  It's a mechanism of last resort, but we've
gotten stuck in a couple of cases.

There is also VM telemetry that we get directly from the Hyper-V on the Azure
hosts.  The guest OS ID helps us bucket the data by Linux kernel version,
without having to do a costly cross-reference to find the image that was
deployed, especially if it was a custom image from the customer.

But notwithstanding all that, my previous comments still stand. I don't think
the marginal benefit is necessarily worth the effort of putting a valid value
in the "d2" field.