Yay for fishing! I heard that fishing used to be very very profitable, but Blizzard drastically cut back prices on them lil' fishies ^_^; Still, I have mine capped at 150 right now.
From what I hear, fishing is useful first and foremost for Hunters. Hunters need constant food for their pets to keep them happy, and fishing provides for that quite nicely. For Hunters usually got fishing.
Second, and this has been true for me, fishing is good for alchemists. We need Oily Blackmouths and Firefin Snappers for reagents. Or is it regents? Bah, how about ingrediants? ^_^; These can be fished up in Menethil Harbor at around 125 skill.
Another reason I leveled fishing was for cooking purposes. I love being an all-purpose player, and whenever I got cooking recipes, I wanted to learn them rather than sell them. So, I would fish, turn the fish into food, and get skill until I learned a skill. This has worked up to level 130 cooking skill by cooking just plain fish, no other ingrediants. Not that it matters, but cooked fish usualyl sell for a copper more to vendors ^_^. Also, higher-end food increases your STA and SPI. So, you get an extra 60 hp for doing nothing! Yay!
Other than that, I haven't seen much profit in it, and fishing for alchemy fish is a lot slower than farming green humanoids at level 30+ for profit. If you want to experience everything though, I recommend fishing ^_^