The question that raises from a process improvement perspective then is “were the first 3 rounds really effective tests?” Perhaps a better solution is not more interviews, but more focused interviews conducted by the people that actually have the knowledge and power to make the decision. (And if the knowledge and the power are divided among multiple people, another great improvement would be empowering the people with the knowledge.)
That strikes me as highly reflective of google’s position of power; from the employer’s perspective, the point where the diminishing returns are no longer worth it is related to the point where they’re losing too many applicants from interview exhaustion. If you’re not google, not offering the kind of pay and such that google does, your break-even point is likely much sooner.
Additionally, from the worker’s perspective, the only-3-interviews rule is an assertion of our power. And, as an added plus, if enough people adhere to it, it will shift that break-even point even for places like Google, and resist the shifting of that burden onto unpaid workers.