Auctually I think attack power wise, some of the peices are equal to peices of the right type, such as comparing 5 str bracers to 10 ap, but you lose out on whatever 2ndary effects the str, or other stat would have given.
like fortified bracers of str+5=Fortified bracers of power+10, same ap bonus but nothing else for the ap version, unless im wrong, havnt compared them in a while.
Or the crit from Agility if you're a rogue/hunter, don't forget. The only time I see anyone wearing AP gear is if they're using it as a filler for BiS gear that actually comes with +agi/+str.
Even in other brackets raw AP gear is very rarely BiS or close to BiS. It only flies when the AP bonus has a large enough impact on overall dps to make up for the forgone secondary stat increases. Off the top of my head the only example I can think of that does this is Outlaw Sabre from Blackfathom Deeps--no stat bonuses, but +15 AP.