As a 70 hunter i have tried many trades , the one i wish i had started with is engineering though , i now have an alt doing engineering and the benefits from it are great , the guns/scopes all sell well , the exploding stuff all helps as well and the robot helpers give you an extra fighting pet for those times when you get mobbed , it does need mining with it though as trying to buy materials would break your bank in time.
leather-working can be good at times but only if you want to spend hours grinding , i know as my main spends most of her time trying to get her trade up to 375 , dam skins take forever to collect and the other things needed like primals well they are no fun either. plus would need skinning skill as mats get expensive
jewel-crafting can be great but again needs mining
Last week i rolled 4 horde hunters and a priest just to get the trades , So far i have the priest at level 11 and doing well with enchanting and tailoring , it works well as all the alts send her the cloth , so she is getting to a decent level.
the B.E. hunter i got to 21 and is my jewel-crafter , doing ok there.
the orc hunter at 11 is my leatherworker and is supplying the others with armour.
the Troll hunter at 11 is my engineer and so far has given the others bullets and guns/sights , again doing ok there.
the Tauren hunter at 11 is my herbalist , still working on her though.
all in all a lot of work and yet i have about 12g after starting all 5 just a week ago , i hope to keep all of them roughly level and with the trades slightly above their level , so far doing all the trades except blacksmithing seems to be very useful , but i may even try that with one alt and use 1 alt as my main miner .
i would say depending on your level it all depends on what you want , if you are 60+ with things like engineering you will see no benefit until you get the skill right up as most stuff is not sellable and will be of no use to you , and selling to vendors will make little gold if any , but the guns/scopes and bullets can be sold and high end sell for a decent profit