You are thinking about when we are 6 months into the patch, but we're not going to be there for a long time - especially not right away.
I am not going to have any legendary upgrade until around 5 weeks into the patch when rank 6 is craftable.
At that point only one legendary will be rank 6 in a new slot if desires, but all of the others will still be rank 4 and i will have to unequip domination gems in order to put them on.
This includes multiple legendaries from each spec.
That leaves three choices:
A: Recraft the same legendary at the same rank into a different slot to be able to use it alongside your gems
B: Choose between legendary and gem
C: Pretend that the legendary doesn't exist for a few months until it gets to the top of the soul cinder queue