clicks can't be tracked or credited, if they didn't happen. that's not worth mentioning. no matter if ppa or ppc, the click is the starting point here.
i'm talking about the case in which a click has occured. whereas under ppc conditions the case is settled, under ppa conditions the time-span between click and action is vulnerable to numerous technical and behavioral distortions. in consequence the origin of a sale is in part impossible to track, so that the action doesn't get credited to the publisher.
fraud is no less a problem under ppa. the difference is, that the threatening potential is now passed over to the advertiser side. if fraud is committed, it could be even more damaging for the whole reputation of the system because of the worse risk distribution for google and its affiliates.
ok, i think we're finished here. we've worked out the different points of view from an advertiser and publisher perspective.
my conclusion stands:
cpc has shown to be the most convenient accounting method for all parties. in terms of risk distribution located right between cpm and cpa, it ensures the needed balance for financial planning aspects and assures, that none of the parties has to refuse the program to a greater extent. the incredible popularity of adwords/adsense on both the advertiser and publisher side and the resulting huge financial success for google confirms that.