As a rank 50 fisherman I will agree that any fishing spot over grade 6 has utterly useless fish; however, on the point of why to take the class to 50 is a simple answer. I make more gil with my fisherman than I do any other class, and because I'm rank 50 I can wear the better gear and boost my perception, gathering and output stats by quite a lot compared to the rank 30 gear. Which in turn means I catch more fish per cast, and get more bites and I don't need to fight the fish nearly as hard. Just because all the worthwhile fish are in lower rank pools, doesn't mean you shouldn't rank your fisherman up past that point.