If you don't see the benefit of fishing, that only means that you haven't done alot of fishing. The majority of the fish you catch in general will tend to be junk fish that you can cook to eat, I would suggest using these fish to level up your cooking skill.
The minority of the fish you will catch are the money makers and are generally used in alchemy, with the exception of certain fish like the deviate fish (which can be fished up in the oases in the barrens)that give certain bonuses that may prove useful in certain niches of game play.
The fish that you will want to sell in order to get rich from fishing are the oily blackmouth, firefin snapper, and the stonescale eel (which is the king of all the fish - try and get this sucker).
If you want to learn more about fishing, there are numerous threads and guides throuout this site that will give valuable information. If you want to learn more about fishing for stonescale eels, I would suggest my thread located in the trade forums titled
fishing for stonescale eels. As far as DE'ing your loot, there are also numerous threads and guides out there talking about that, I would specifically like to draw your attention to this thread in the trade forums titled
How to make lots of money with DE (guide). This guide will teach you the best way to buy cheap materials for DE'ing for profit or for use in enchanting.
As far as a second profession goes, anything will work fine. I would suggest starting with a gathering prof such as mining or skinning, mining preferably so you can sell or give ores to your friend when he's busy crafting and doesn't have the time to go farm them himself. Later on it would be a good idea to ditch mining for a more lucrative gathering prof such as herbalism.
At a high level, or once you are rich then I would suggest ditching the gathering prof for a second crafting prof such as tailoring, or preferably alchemy. Alchemy seems to have the most recipes for which the mats cost less than the pots that you can make, so you won't have to worry about gathering them, you can buy whats available on the AH for a resonable price and sell the pot for a profit.
Coupled with enchanting, which early on has a huge money sink to it, but later on has the most recipes that you can charge an arm and a leg for and everyone can use, you'll be able to buy your lvl 40 mount, your lvl 60 mount, and your lvl 70 mount (once TBC is out).