It's like writing copy about hair dryers, one comment-shop explains, "The only difference is that this hair dryer is a political one."

By Olga Khazan, The Atlantic, Oct. 9, 2013

The principle point at the end actually applies everywhere in my opinion. Reams of agitprop (or P.R., or talking points, or whatever you want to call it,) even without abuse, discourages lurkers (often interested in learning some nuance on topic rather than reading passionate advocacy and debate) from participating:

[....] some Russian opposition journalists point out that this trolling creates a chilling effect on the few independent media outlets that remain in the country. Finding themselves drowned out by paid propagandists, as opposition activist Vladimir Volokhonsky told the St. Petersburg Times, everyday readers stop responding to news articles entirely:

“The effect created by such Internet trolls is not very big, but they manage to make certain forums meaningless because people stop commenting on the articles when these trolls sit there and constantly create an aggressive, hostile atmosphere toward those whom they don’t like. These include commentary systems on the web sites of every major media outlet in the city that the trolls began to occupy a long time ago and react to certain news with torrents of mud and abuse. This makes it meaningless for a reasonable person to comment on anything there.”