Isn't the 8% figure the number of tickets redeemed...not the % made available?
And I really do think the #s we have seen are misleading.
Say I call AA once for a good redemption value and get it 1st try. Say I redeemed 25K for a $500 ticket.
Then I call DL for a similar ticket. Nothing. I repeat this five times over the coarse of the year. Nothing. Finally, I give up and use 25K DL miles for a $150 ticket to STL.
My DL miles are basically worth 1/3 my AA miles but the number would still look the same.
As for my experience (about 5 million miles in 10 different programs) I rank them as follows:
1. AA
2. UA
3. DL
4. US
5. NW
6. CO