If you want to do leatherworking/skinning DONT do it thinking you can make your gear. Thats the wrong reason to.
Personally I am a mining/goblin engineer and use it a lot. Make my own ammo (if you use a bow you can still change the thorium bullets to arrows from an NPC in the main towns). Also for the most part the NPC purchased ammo usually costs less than its crafted equivilent anyways. What is nice is te grenades and other nifty gadgets I can use. ~400dmg grenades with an AOE stun for 3-5 seconds (depending on grenade) it is wickedly helpful. Plus a few other odds and ends. As well as the repair bot. Though I have not used one yet (havent been to MC). As for the jumper cables, my initial mistake was going eng just for that (I used to be an armorsmith) they are still pieces of junk and rarely work, even the XL ones. However find a friend that is gnomish and have him make you the net launcher and things like that.
remember that engineer can also make scopes and lots of guns as well. Plus being an engineer makes the darkmoon fair easier to get tickets for ;)
As for skinning and stuff, its helpful, its honestly nice for grinding/farming cause your income is from teh mobs you kill, downside is you cant skin humanoids :P which actually drop silver and cloth.
And there is really nothing wrong witha gathering profession, oyu just dont want both at once. The only time you dont want to scan for herbs/mines is if your looking for a specific mob, otherwise you dont need to worry about it too much.