CPA = Great for Advertisers, Crap for Publishers
CPC = Ok for Advertisiers, Ok for Publishers
CPM/CPV = Crap for Advertisiers, Great for Publishers.

Ad copy on the internet is all about a few things:
1. Intent - if you just want your name known by as many people as possible then CPM is great, if you dont have a narrowly defined audience or do not sell product then CPC is great, and if you are selling something the Afilliate programs or CPA is the best. - a good mashing of these works even better, but it will take ad testing to find out what works with what geographic, demographic and psychographic markets.

2. Message Sender - Is the advertising conveying the right message? Are users to your site paying attention to it? If not it isn't your fault as the publisher, its the advertisers fault for making crappy ads. (see GameRail - no defunct). Ads must promote AIDA or you're kinda just wasting your time and money.

3. Message Channel - how many other crazy flashing ads are there on your page? is the ad somewhere were the normal user is looking? Is the ad taking away from your content? (i.e. if you are giving away free templates and the ad says 'OMG FREE TEMPLATES - CLICK NOW' and shows before your template download links then users are more likely to click the banner and move on. This is a common practice amongst adult and warez websites).

4. Message Reciever - Did the ad at least get the attention and interest of the user? If not they may not be the target demographic or user, which is fine. Too many losses here and advertisers wont come back.

For those who don't know AIDA its:
Attention - get them looking
Interest - get them looking closer
Desire - get them wanting your product/service
Action - get them to take your product/service

CPA ads basically give the advertiser a free ride through the first 3 steps, not holding them liable since the cost to run such programs is negligible compared to a highly effective CPC/CPM campaign.

In short: CPA gives Advertisers far more than it gives publishers. It allows them to FOR FREE get a users Attention and Interest. And without risk try to sell them products. It also gives valuable feedback on why users are not purchasing. Things like which ad copy works better in which markets, bounce rates etc.. and they get all this on the Publishers Dime.