I thought people had moved on from using ratings like that because the MPAA was suing someone for using it's rating system for fan fiction.
Of the fics I've seen in my corner of Fandom, some folks have moved on to various other rating systems, but most still use the MPAA ratings (because people are familiar with them).
Either way, I do agree with you. I'd much rather see what the story involves rather than a simple rating.
That, too, but what mostly annoys me is when people use the same rating for all their fics, so you can't tell from that if it actually contains G-type content or R-type content.