Google recommends to delegate composition to GLES instead of HWC when screen isn't updating to conserve power, as stated on this page.
Current implementation of hwcomposer has flattening of layers if after some time there were no updates of frames, but it uses writeback connector. Not every device has a support of writeback feature, so some sort of fallback should be provided.
It is possible to fallback to client composition in case if writeback isn't available. This is used to reduce power consumption since squashing layers into a single layer on GPU and then using that buffer is more efficient than loading drm device.