I just started working on a triad of characters to address my crafting needs, and I can't seem to find a reason to worry about mastering the gathering portions (prospector, forester, etc) since it only seems to provide the 5% crit chance. The benefit of mastering the item crafting professions seems obvious enough, but am I missing something for the gathering professions?
You cannot process resources of the next higher tier until you finish the red portion of the lower tier and move into the yellow. For instance, you cannot prospect Barrow-Iron/Silver (Journeyman) until you have prospected enough Copper/Tin (Apprentice) to completely fill the first red tier of Apprentice prospecting and start the second yellow bar. This point also grants you a title as a prospector.
The second restriction is that you cannot start the yellow section of any tier until you fully master the tier below. So, you cannot start the yellow section of Journeyman (and thus not start Expert at all) until you complete the yellow section of Apprentice.
I know, I said I quit, so why am I posting? 'Cause there ain't no Blogorette patch.
Yes, you have to finish one tier of Proficiency to open up the next tier, and you have to finish one tier of Mastery to open up the next tier, but you do not have to finish any tier of Mastery for Proficiency to progress.
For example, once you complete Proficiency (red) in Apprentice 2 things happen: the Journeyman Proficiency tier opens up and the Apprentice Mastery Tier opens up. You can choose now to continue with Apprentice and Master it, or move on to Tier 2 Journeyman and work on that.
Once you reach proficiency in Journeyman, Expert proficiency will open, but Mastery will not until you've completed Mastery in Apprentice and Journeyman first.
So it's possible to be Proficient in all 7 Tiers without ever Mastering any of them.