CRAFT MOAR!!
Seriously though, I am currently CRP 26, BSM 22, ARM 21, GSM 25, LTW 25, WVR 25, ALC 26, and CUL 21 (as well as BOT 22, MIN 23, and FSH 22) and I intend to hit lvl 50 on all crafting classes in the same week (and knock that self-styled Queen of Crafting who only has 3 classes off her throne...

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If you have the time, and enjoy crafting there is no reason to not level them all, and if you intend to level them all, there is no reason to not level them at the same time. You may run out of inventory space on your retainers, but you will save some in your armory chest
