Imagine what would happen if you suspend while usb cable is attached. You don't really know whether pc has mounted the file system (considering you're using mass storage) and if the pc is transferring files to device, since with mass storage we need a 2 second poll for media change and there's no way to differentiate those IRQs from actual _data_ coming to device. They're all endpoint IRQs.
If you forcefully suspend at that point, you're risking corrupting user's Data.
Also, we can get pretty much the same power consumption with cpufreq + cpuidle + runtime pm since idle consumption is HW characteristic, not SW. If we don't reach that level, it only mean device isn't idle enough and there's some cripple app waking up the processor.