I don't understand why -in Yosemite- they eliminated the small rotating TM icon when it was backing up!
If you have "show Time Machine in menu bar" turned on, you should see a clock icon with one arrow facing downward. When Time Machine is active, you will see a second arrow facing up that meets the first arrow.
Making a thread over such a trivial matter, that does not impede your continuing use of the OS should bug you more, in my opinion. It might behoove you to focus on more substantial matters of operation.
What BasicGreatGuy said. Besides, the icon was eliminated in Mavericks. At least in Yosemite, the little hash mark showing that TM is doing its thing is at least a little more noticeable.
Having to update the icon a number of times a second was obviously preventing the CPU from sleeping properly which in turn was using significant amounts of power. The removal of that animation also coincided with the introduction of "Apps using Significant Energy", so I guess they didn't want to "name and shame" their own app.
That would be a reasonable explanation. Even minor animations can have surprisingly high impact on energy usage as the OS can't keep the GPU in the low-power state all the time.