I've always been puzzled by this too. First Aid is really the only secondary skill I've ever bothered to level up (because it's necessary for almost any class). Cooking and Fishing have never held interest for me. I tried both of them out-- on my hunter and warrior especially I thought they would come in handy. But the time (fishing) and cost (cooking ingredients) involved in both never seemed to compare to what I got out of them.
Players in the thread claim you can get a lot out of fishing: special food, alchemy ingredients and recipes, and a chance at treasure that you can't get otherwise. But for the time commitment or cost, I've always found it's easier and cheaper to just pull what I need out of the AH. Fishing reminds me of crafting in Dark Age of Camelot-- if you ever played that game, you'll remember that higher level item crafting took longer and longer casting times (until, at high levels, it took like five minutes of meter watching to make one item). Both cooking and fishing seem like neat gimmicks, but in terms of gameplay, I've never really found them necessary or even interesting.
I do have a Nat Pagle's Extreme Angler FC-5000 sitting in my Shaman's bank, however. Is there a good reason I should get it out and start angling? And if so, is there any way to make fishing more interesting?