Basic idea is that you have one job and the 2 subclasses leveled to the appropriate levels. Beyond that no bad thing about rolling alts. So you should have atleast 3 classes leveled one to max and two to what you need for them to be effective endgame. As for crafters you need most of them on a single job... it goes the same for botanist/miner. Also for crafters and gatherers they share the accessories often.