In a developer Q&A, Bioware said preferred accounts will have 4 quickbars and 6 char slots patched in. I assume this will be when 1.6 hits, so buying quickbars now wouldn't be worth it. Just sub for a month to level faster and when that's over you get these extras.

I managed to get artifact equipment and section x account unlocks off GTN the other day for one account, so they are slowly coming up at better prices for extra accounts you don't want to permanently sub on.