Because "signing" comments is not customary here, doing so signals a certain aloofness or distance from the community, and thus can easily be interpreted as a passive-aggressive assertion of high status. (Especially coming from Luke, who I find emits such signals rather often -- he may want to be aware of this in case it's not his intention.)

I interpret Silas's "Not necessary" as roughly "Excuse me, but you're not on Mount Olympus writing an epistle to the unwashed masses on LW down below".