Some key items are useless. Sheeting in Smithing comes right to mind as does Fletching in Clothcraft. Some are even worse than useless like the boltmaker, which eliminates item multiplication through HQing. Some are useful for leveling that craft, I used Concoction to level Alchemy on stackable drops rather than potions and ethers and ... forget its name but it lets you make the Fragrant Ram Skin needed to make, and level on, the High Breath Mantle.
I respectfully disagree, the majority of the "synth multiple at a time" saves you money on crystals and more importantly (to me anyways) time. The exception would be boltmaker, get it only if you have guild points to burn or never expect to HQ a bolt synth.
Sheeting allows you to make 6 sheets at time. Saves you 5 fire crystals and 84% of the time per synth.
Tanning is a must if you're making leather in the field. For every stack of vats, you carry 2 less stacks of dark crystals and 2 less water.
Fletching is useful for all of the above reasons if you're making arrows.
As far as the Purification/Ensorcellment KIs go, I find them to be of limited utility. Prolly want to get in your main craft just for completeness sake, for subcrafts usually skippable. Exceptions might be made in a few cases (Gold purification lets you make RR earring at ~40 goldsmithing) Others do research and use common sense.
Regarding the armors, get apron for main craft (or two pending update) and the head/glove item if you think you'll use that subcraft. (I also like to wear the sunglasses in town ^^)