2 things:
1. -> One might argue it's much easier to put points on the board when your drives start at or near midfield than when you get the ball at your own 20. Field position not only matters, but with a ball-control offense, it's critical.
2. -> I took a look at the stats for NFL kickers from 40-49 yards. I took the NFL kickers who have attempted FGs of 40-49 yards, and the average is 66.67059%. Which isn't too far from the 70% figure you used, but does not take into account the average FG% from 45-50 yards exclusively. Had those stats been broken down by the NFL, I think that average would drop dramatically. Since the earlier poster said punting from the 30, that makes for a 47 yard FG. He later said the 35 yard line in the same post, which would make it a 52 yard FG. At 50+ yards, the NFL average is 60.037%.
What it comes down to is: Would you rather take a 60-67% chance of scoring 3 points while giving the opponent a 40-33% chance of getting ball at midfield, or do you like the odds of that an opponent won't be able to march the ball 80+ yards and scoring against our defense?
I'll take field position every time.