Rodent-proof compost bins are wonderful. They should be encouraged. A three bin set up is useful for keeping composting materials contained and if it is rodent-proof, food scraps can be included as well. Cold composting with three bins always gives the gardener a place to put still-composting material when it is time to collect the finished compost below. Often the third bin is described as a place to put finished compost, but frankly, when I have finished compost, I always have other places to put it besides another bin. Still, a third bin is good for the extra space it provides.