dhenry:
So why the complex timing arrangement with two timed interrupts keeping the processor busy all the time?
Not really sure about the "interrupts keeping the processor busy all the time" part. Interrupts are useful precisely because they do not keep the processor busy - they interrupt the processor only when certain conditions are met.
Interrupts can keep the processor busy. If they fire too frequently and/or take too long, the CPU will have only little time to execute loop() code.