Is there a purpose to off-hand crafting? or is it just to take advantage of a different stat? IE for GSM is there a hidden bonus to using the grinding wheel on refining gems?

do the stats on the off-hand effect the primary? vice-versa? (specifically craft/mg. craft)

for gearing up on crafting, what stats should be prioritized? IE: should i always look for craft+? or is stacking the primary stat for that tool better? at what point should i start getting control? etc.