PPC management has become very sophisticated and requires a lot of specialized knowledge. In most cases, you need an account manager that is totally dedicated to PPC advertising, with lots of specialized training and experience. Unfortunately, certification does not ensure competence, nor does select an agency guarantee superior performance.
As a member of AdWord’s partner community I see many threads where agency accounts managers are totally focused on how to spend as little time as possible on managing accounts. Many decisions are made based not on what brings the best performance, but what requires the least amount of time to manage. Many managers prefer to use broad match keywords, not because it provides better ROAS, but because it generates a lot more impressions and clicks. For them, conversions and profits are the business owners’ problem, they just provide traffic and broad keywords provide the most traffic. And these are all certified managers working for Partner agencies.
Not all are like that, I believe the vast majority of badged agencies are very competent, and some are true superstars win it comes to performance-based campaign management.
When hiring an agency please bear in mind that it is an inescapable fact that nearly half of all agencies are below-average performers. You may have to try out several agencies before you get an above-average one, and perhaps a dozen or more to find a true superstar agency.