"I have not failed. I've just found 10,000 ways that won't work." - Thomas A. Edison
If Thomas Edison were to run PPC what approach do you think he would take? Would he wait until a campaign stabilizes or would he constantly push the envelope with the hopes of getting better.
Attention given to a PPC account can produce diminishing returns. At first the account has to gather data, then we optimize. Then the account expands until it reaches a optimal size and spend. We can get a nearly perfect keyword list and a nearly perfect campaign structure... but not everything can be perfect.
But there is one component that can never be perfect... your ads. This is why we continually look to test every possible ad idea imaginable. They all shine, which ad shines the brightest? If Thomas Edison was running PPC, he'd test 10,000 ads before he landed on his best ads. We tend to agree.
Now there is a limitation to this metaphor. Obviously while Edison was looking for the perfect variation of carbon filament to complete his design of the light bulb, the light bulb did not work at all. With PPC, the account is working already, the only question is, how brightly can that account shine?