I view that as a more intrusive option to someone's time compared to simply hitting an "x" on a banner message. I'm operating on an assumption that the greeting is sent with the same mechanism as the update notifications. Since this is a community driven project, if you'd like to see the subscription or opt-in/out methodology, maybe submit either as a feature request or possibly look at the source and see if there's a mechanism that could be leveraged or write a module and provide it back to the community.
Since it's a minor action to click the close "x", in my opinion, I don't think it would justify the lines of code to enable/disable notifications based on say "holiday" criteria vs. update notifications.