It's deceitful. Usually it's done to make it appear as though multiple people share the same point of view, or at least more people do than otherwise care to comment. The goal then is to persuade readers based on frequency with which an opinion is hed.

It's used as a cover for spam. If you post the same message 20 times, I think it's reasonable to view doing so as spam meant to drown out productive conversation. But if 20 different people do it, that's not spam that's a whole bunch of people expressing an opinion in the same way.

Ultimately if your concern is "deceiving the readers" (though I am always surprised by how many people here are concerned with these nameless, faceless 'other' readers that are clearly downtrodden and in need of sympathy and protection) then it seems like you should also not want deceipt by commenters.

Just as you also wouldn't want deceit by forum community members who post misinformation on deal threads in order to prevent people from taking advantage of deals and keep those deals alive longer for themselves.