A client of mine talked to someone, who based on reputation should know what he was talking about. He recommended changing to separate accounts for each site so that the sites with worse CTR don't drag down the performance of the other campaigns.
Does Google use an overall CTR metric for determining delivery and position on separate campaigns?
At first glance that seems like a bad idea, but I guess it would lead to more interest in the Client Center and more people going through the certification training and ultimately smarter ad buyers but the whole thing sounds fishy to me.
As for the My Client Center, the advantage is that each site owner will be able to access their account without being able to look at the other accounts. It also helps if you ever want to spread your wings and create more campaigns per site. We have multipul duplicat campaigns to test site targeting. This also helps if you track the leads and enrollments through the URL, then you can test two different ads in different campaigns for better diagnostics.
My client center also allows you to apply for a Google Advertising Professional licence. It also helps if you want to run more than one of your sites with the same keyword list.
