Yes, you can buy everything you need from Vendors(Thread, dye, etc) and from the AH(Leather, other mats).
Is skinning necessary? No.
Is skinning useful? OMG yes! I've made SOOOOOOO much gold off the leather I've auctioned off on AH. 99% of the leather I've put up for auction is bought out within 10 minutes of the auction beginning.
Besides, it's a lot cheaper to skin that Devilsaur than the buy the Hide on AH. On my server(Mannaroth - Alliance), Devilsaur goes for about 5 gold a piece. No, not per stack. EACH. A stack can cost you up to 60 gold.
Sure, you have to be high level to kill the devilsaur to skin, but you also have to be high level to wear the Devilsaur gear.
FYI, I'm a 300 Skinner and a 300 Elemental Leatherworker. I have, at one point, posessed over 2600 gold, mostly acquired through leather sales(both finished leatherworking products and raw leather), and continue to make gold just curing leather, among other things. Curing and the like falls under Leatherworking, yes, but it's much easier to just skin the many many beasties you kill over time than to fork out a gold for a stack of basic leather (Light, Medium, Heavy, Thick, Rugged).