Participatory cloud computing - help cure cancer?

How come so few donate their free resources to volunteer computing projects? Depending on the exact computer configuration, the difference in system power consumption between using and near-idling the CPU (~35W-80W) or GPU (~30W-100W) can be reasonably small. The difference is usually smaller than the difference between a power efficient (25-60W) and a basic system (100-200W).

Hence in my view, it usually worths it to put your free resources to use in some way if your daily work does not strain your machine continuously at 100%.

There's an a project of special interest that has similar aim to what I have invented earlier: It is special in that any participant can freely use the communal grid for their own purposes. Prioritization is fair - it is based on previous record of contribution. This type of project is also useful for the selfish kind, as if you think it over, you can eventually get back the contributed CPU time and energy, but at a much faster pace. It would still net out positive even if each user eventually used up all the CPU time one has contributed, as work would always be done much faster. Think of solving an iterative optimization problem or other algorithmic tweak in rapid turnarounds instead of running each batch locally for days.