I think the below one hints to it being for compat reasons but you're right no explanation as to what possibly could go wrong if they did it automagically - >

I think the below one hints to it being for compat reasons but you're right no explanation as to what possibly could go wrong if they did it automagically - > Existing Applications and Loopback Fast Path The default behavior of the TCP loopback interface is unchanged in Windows Server 2012 thereby preserving compatibility. Microsoft being overly compatibility cautious as they should be - it's worked well for them. But on the other hand they should have pondered on what sort of compat issues might pop up if they defaulted to setting the flag for everyone and disabled it on per app blacklist basis or something like that.