This would be a great addition to D3 as we wait for D4.
The only thing I dislike is them taking the space of regular gem sockets. Instead, I would rather we have a rune pouch which will have three grids. A triple connected row, much like your armor example above, a double connected and one single row (equating to pants and helm), and than a triple non-connected row, which would compromise the jewelry sockets. In all, you can have 9 runewords equipped like above, but it leads me to allow this following addition:
MODIFIED LEGENDARY GEMS USEABLE ON REGULAR GEAR SOCKETS! Simply put, when leveling a gem to level 75, create a cube recipe that costs 300 forgotten souls, 1,000 deaths breaths, and 100 of each bounty mat in order to modify a legendary gem. By modifying the legendary gem, you unlock a level 75 gem bonus related that gems theme, as well as the ability to equip that legendary gem in ANY SOCKET.
Yes, that would be power creep, but honestly, NUMBERS ARE NUMBERS, and this would allow a more in depth optimization, which is truly all I care about in my ARPG's!
*PS Bring back Charms in a similar fashion to my rune pouch idea!!!