Runic Spellthread requires Primal Nethers. Do you really want to have to get a group to go back and run Heroic Mech/SP/Whatever for the umpteenth time? How about running Kara again so you can have enough badges to guy a couple epic gems to upgrade your gear.
Blizzard has left you the option to go back if you want to, but also given you an easier option to move ahead.
I normally farm my mats and then find a crafter, but if people had to go back to run old content just to get those augmentations to their gear, you'd see hardly any business and a lot of people sucking it up with whatever green and blue gem they can get without having to go back and run those old heroics.