1. Whether the pricing metric is CPA, CPC or CPM, it’s still a lever that you move up or down to get more or less traffic and hence transactions. Over a portfolio of keywords, you’ll have a set of possible CPAs that you need to balance out to hit your blended CPA target for the whole campaign. So you have the same problem of low CPAs, but limited converions on some keywords, and high CPAs but lots of conversions on other keywords. Working out the best way to manage this to maximise transactions at a given CPA is still a complex mathematical challenge. Efficient Frontier’s technology is well suited to solve this problem.
2. Secondly, Google hasn’t had much uptake from its publishers for the Pay Per Action (PPA) model. I understand that volumes are extremely limited.
3. Thirdly, I don’t believe Google’s Conversion Optimser will be useful to advertisers who require any level of sophistication. I’ve elaborated on this in another post about the conversion optimiser. You’ve also given an excellent analysis of it yourself in your blog.

Google now offers CPA pricing and can optimize to a max. CPA goal. That could put pressure on SEM agencies who optimize to CPA goals.