I have to admit, athletics definitely isn't worth going past once you hit 7 or 8 if you intend to use medium armor (9 to 13 weight).
I was fighting San in the duel server with a 12/27 STF and he asked me how much athletics I had. I answered "9" and he seemed surprised that I had so much.
To give an idea I was wearing a Haubergeon, Mail Gauntlets, a Kettle Hat and Splinted Leather Greaves.