as long as you don't have more than a dozen, it would probably not waste much electricity at all.a fan does not have much mass so starting it would would not be too much burden.
it's not just the fans on the GPUs.
put a Kill-A-Watt on a miner sometime, and compare the power draw in the first five minutes of operation to the draw at steady-state.
write up a lab report for me and ill belive you. you fail to state the state the "miner" was in before you started the meter. stopping and starting a mining operation, as long as all the data stays in ram, and the gpu just stops doing numbers, hardly any energy would be wasted by not mining, other than keeping all the equipment idle. with 3-5 gpus the savings would be great because you only have 1 set of ram, cpu and mobo and 3-5 gpus that won't be working. with only 1 or 2 gpus on a mobo i could see it as being a waste, but with 3-5+ i could see it working assuming the original assumption was correct.
i have two GPUs/mobo. the reason for that is that it's a much
more cost effective build. the PSU to run a couple of 5870s doesn't scale at twice the dollars to running four - that
PSU is much
more than twice as expensive. ditto for a two PCIe-slot mobo vs. a four (really five) slot mobo. and etc.
hardware cost doesn't scale arithmetically - it scales geometrically.
so yes, i can see it working for a 3-5 GPU setup too. but the upfront costs would be ridiculous. talking about the comparatively minuscule differences in profitability due to starting and stopping vs. not
starting and stopping would be a waste of time - if one took all
costs into consideration.