Point of Fishing Skill Beyond Max?

Hello, Icecrown server WotLK here.

If I have 450 fishing, what is the point of using lures and special hats/fishing poles/potions to increase the fishing skill beyond max level? Maybe on retail you still had a chance to fish junk in Northrend open waters at 450, but I never catch junk at 450 anymore.

Does having a fishing skill of 550 increase the drop chance of rare fish like Old Ironjaw or the Sea Turtle mount?

That's how it used to work on retail yes, the more skill the higher chance you get loot that is rare.