While I think it is unfortunate that anyone would this, I don't think it changes much. It doesn't get them more revenues, since users who install the plugins will quickly learn the truth, and it doesn't affect me much since I never settle on a plugin based on reviews but on trying it out. It does, however, affect the search results, making the search engine flawed and thus slowing down the trial-and-error process.

It reminds me of a stripe from xkcd...

http://xkcd.com/1098/

That illustrates the rating problem rather well, I fear.