Why? Google said there are "pitfalls around targeting average position," and linked people over to Chief Economist, Hal Varian's post named understanding the Average Position metric from a few days ago. But not everyone agrees that this is a good thing. One advertiser in the WebmasterWorld forums said:
This is really bad news for the way I handle adwords - the top positions convert the best for me, while the lower ones do not. Hence with a limited budget I use position preference to make sure that when my ads appear, they do so in the best converting positions.
This is going to increase my cost per conversion and overall adspend considerably - great for Google, not so good for me!
In any event, this person can still do a position preference like bidding style but they have to use the automated rules feature in a interesting way.
(1) As of April 5th, position preference can no longer be enabled for campaigns through either the AdWords web interface or the API. Campaigns already using position preference will still have it enabled, but if you turn position preference off in one of your campaigns, you won't be able to turn it back on.
(2) Starting in early May, Google will begin disabling position preference for any campaigns still using it.
(3) After you disable position preference (either manually or when the feature is retired starting in early May), the manual maximum CPC bids for those campaigns will be the bids position preference used most recently. Position preference tries to raise or lower your bids to target the positions you specify. So using the most recent position preference bid as your manual maximum CPC should minimize disruption to your traffic.
For more details see this help document.
Forum discussion at WebmasterWorld.