I ... still do not see the use of those skills. Mainly, because of another skill called Flawless Synthesis. For 15 CP you get a guaranteed 40 Progress on the item. When you would be looking at 15 Progress on a Careful Synthesis and 17 Progress on a Basic Synthesis, getting that doubled (I presume that's the impact that doubling your Efficiency for a few turns would have) does not provide as much guaranteed progress as Flawless Synthesis.

In working on the Crafting Log for each craft (except Culinary and Alchemy) so far, in going from 30 to 40, I have used Flawless Synthesis to great effect, always HQ'ing those special recipes, and I've not used those Brands once.