I'd say the usefulness of aggregators lies in the extremes - if the aggregate score is 80-90%, that's a remarkably wide range of people saying it was good, so clearly it has a broad based appeal and you'll likely enjoy it too.
True though there are some genres of movies on Rotten Tomatoes that get consistently over-rated compared with their relative merits. Pixar movies and disney-esque animation in general tend to get higher reviews than they probably deserve in many cases. For example Wall-E was a very solid movie and I enjoyed it but it got a 96% on Rotten Tomatoes. You'll never convince me that it was THAT good of a movie. I would have put it somewhere in the high 70s or low 80s. Maybe high 80s at best which is where the audience score was at 89% and even that is a bit high. Totally worth a theater ticket but not exactly best picture material. There are some that deserve ratings in the 90s but those should be much more rare than they are.
Fortunately I'm aware of this fact about RT so I can mentally adjust but it's kind of annoying and makes it harder to separate the good from the great and sometimes results in movies that should be skipped getting decent recommendations.
Similarly, if something is ranked at 10-20%, that's a remarkable consensus that it's bad.
Agreed. I've never seen a movie rated that low on Rotten Tomatoes that wasn't indeed a hot mess.