FIRM greens stimping under 10 would be easier to acheive agronomically as it would be lesss stressful on the grass.
More interesting pin placements could be used instead of the near flat ones demanded by greens above 12.
Firmer greens would make approaching from the rough more difficult as it's not the speed of the green that repels shots from the rough, but rather the firmness. The speed of a green would only save ball from rolling say six feet less on a 36 foot roll, but the firmness could make the ball bounce over without any roll so the speed wouldn't matter.
In the long run, if green speeds were found to be appropriate again under 10, more interesting greens and contours could be built again, forcing the player to actually seek an angle with his tee shots.
Also, with MORE slope and slower greens, there is more variance in speed between an uphill and a downhill putt, requiring MORE skill, so I think better putters would actually benefit or at worst , see no change.