17 had 22 birdies all week and 18 had 11, the two holes with the least birdies. That meant that coming from behind the tournament had to be won before the last two holes. The main drama on 18 was whether the leaders would bogey.
The problem with your argument is that using over and under par to calculate a golfer's position relative to other golfers who have played a different number of holes a completely artificial construct that has little to do with reality.
If the field was averaging 4.7 on 18 (which it was) then Rose's four on the 72nd hole did, in fact, gain about 3/4 of a stroke on the field.
Calling it a par five would have changed NOTHING about the math of winning a tournament.
When you say, "The main drama on 18 was whether the leaders would bogey," you are correct. But calling those bogeys a par wouldn't make the actual drama any more intense, since everyone watching the event had a pretty good idea what the odds of make 4 were.
Just like the two par fives on the back nine at ANGC, 18 at Merion is a half par. The fact that the USGA like half-par holes to be OVER par instead of under, doesn't change the math. We all know that professional golfers about as likely to make a 4 on either 13 or 15 at Augusta than they are on 18 at Merion.
FWIW the stats for 2013 show ANGC #15 at 4.6396 and ANGC #13 at 4.7532. Merion's 18th averaged 4.7068
Calling a two a par four and the other a par five changes nothing about who ends up with the trophy.
K