Efficiency doesn't mean profitable. Traditional miners require profitability while this system would only require some unknown level of efficiency. The equipment costs is basically negligible because you are using idle capacity of existing hardware. The marginal revenue also doesn't need to beat electricity costs because the people paying the electricity are not the ones who are receiving the revenue. For example if every $1 of electricity generated $0.90 of revenue, that would be equivalent to the site paying a 10% fee on the micropayments they receive. That would likely be perfectly acceptable to both sides.