Very.
There's a reason that everyone is telling you the same thing. It's really true. The level 15 skills make doing literally everything else way, waaaaaaay easier. It's also pretty quick and easy. Vendors in houses will sell you all the mats needed to level everything to 20ish for pennies.
That said, if you REALLY don't want to do it you'd probably be okay with skipping Blacksmith and Weaver. Both are important later on, but Ingenuity and Careful Synthesis 1 are less useful at lower levels.