RT is supposed to be a quick way of people checking out if a movie is worth watching or not. In theory, that's great. The problem starts when the consensus doesn't reflect the audience's opinion on a movie. That doesn't happen every time, but it happens often enough for it to be a very flawed and untrustworthy system.
If it works for you, great. It's all about finding out what helps you with your choices. Personally, i think critics screw up way too often for me to put any sort of credibility in what they say.
I haven't seen SS, but with BvS, for example, Rotten Tomatoes is basically telling people the movie isn't worth watching. That's what a 27% tells you. It's not an inviting number. It's a "stay away from it" number. Is it accurate though? Is it a good advice? I'd say AT LEAST 60% of the people i know liked the movie. Not too long ago, i think the like/dislike ratio on twitter was 3 to 1. Just checked the BvS poll and around 70% of the users gave it a positive rating. What this tells me is that a lot of people disliked the movie and it has a lot of problems, but it was still entertaining for the majority. So, i don't think a "not worth watching" is really a good advice.
Sharknado is a better movie than BvS, according to the critics. It's difficult not to see their opinion as a bad joke.
The consispiracy theories are pretty ridiculous, but they're motivated by the fact that so many horrible and so many underwhelming movies get at least a decent rating and a movie like BvS is treated like something you should avoid at all costs. In all honesty, i'm not crazy about BvS and i was very disappointed by it, but there's no way i would consider Superman Returns, Thor: TDW, Iron Man 2, TIH, Batman Forever, among other stinkers as being much better movies than BvS like the site suggests. I also can't agree that BvS has the same artistic and entertainment value as Green Lantern or Superman III. According to this amazing site, you're better off spending your money on Daredevil than BvS. How can i take it seriously? It's difficult.
Sadly, these couple hundred of reviews actually influence the opinion of millions. How many people decide not to go see a movie after reading that it sucks? I'd say a large number.