I'm confused. I don't understand how this would be abusive outside of uselessly popularizing reviews.
I can see how this button could help players figure out which reviews were more popular--whatever a popular review means. This feature won't change the amount of reviews a server needs to get higher ranked on the lists, and if you require ppl to sign in before they hit that +1 button, then that should cull the amount of fakes you get.
Even if a super lauding review gets 1,000,000 -1s, or a scathing flame gets 5,000,000 +1s, all this really is is a review popularity contest. And if you think about it, if one (good/bad) review gets a thousand +1s, there's a really good chance that someone else will write another review with similar (good/bad) content anyhow (which will also get a few +/-1s as well).
tl; dr: +/-1s don't affect the number of reviews a server receives, and thus doesn't affect its ranking on RMS. A popularly +/-1'd review will more than likely already be echoed in subsequent reviews as well.