While I personally haven’t noticed comments being on or off as useful, just because a signal could have multiple factors doesn’t necessarily render it useless.

For example, a Yelp rating itself is useless until you also combine other signals such as the number of reviews, the type of restaurant, and the popularity of Yelp use in an area, and then I find Yelp ratings potentially very useful. Same with Amazon reviews, Rotten Tomatoes scores, or any scoring or review system.