It's a hint to users that reading mode may provide a better experience. When we introduced it, reading mode usage went up; without the (very subtle) animation, users weren't aware when the feature would be useful.
The real problem here is that, as with a lot of other aspects of UIs today, most of your users probably have no idea what that icon or indeed a lot of the others is supposed to mean. If it was just plainly marked "Reading Mode", you might get the usage increase without wasting countless hours designing an animation and having it, in turn, waste the energy of your user's machines and then having to spend even more time and energy "optimising" that animation.
Stop making UIs consisting entirely of vague mystery-meat-navigation-buttons-some-that-don't-even-look-like-buttons and none of these problems of discoverability would exist.
Of course usage will go up when you animate it. That's like saying people click the Ok button more when you put it on a popup dialogue. The question is: do people keep using it when you don't animate it. Were they using it because it was animated, or because it's something they like to use?
Are you suggesting they're only clicking it because it shiny?
If they didn't like it they'd stop using it after the first click and the numbers wouldn't appear to go up more than a single little bump. Clearly it helps people to have that cue there.
Still it shows what's done just to make somebody's bonus (or just prestige in the meetings?) thanks to the change in the telemetry reports. Luckily both Safari and Firefox don't animate it.
And if I'm not wrong, Safari was the first to have it, years ago, at the time Jobs was there. Never animated.
Anyway, if the feature is useful, people will use it. If it's not, they won't. Not making the whole experience a "big ad" is an example of a taste. Microsoft managers however really like everything moving like in ads including the tiles in the "start" menu. Luckily they at least can be "unanimated," I hope that feature remains (but unluckily I believe I have to do it one by one).
People will use it, but only if they know it's there. I've met a ton of Safari users who have no idea what that thing is or even that it's there in the first place.
They tend to love it after I pointed out to them. Microsoft is probably doing the right thing here.
We get great feedback from users who discover the feature and want it to be discoverable without being obtrusive. If you haven't seen the animation, you might be overestimating it - it's extremely subtle and tasteful, just a tiny flip of the pages in the reading mode button when it goes from an inactive to active state.
Even if you insist on the "ad" it still needs to be displayed only to the users that never clicked it, once anybody clicked once, you aren't helping him "discover" it.
Stop making UIs consisting entirely of vague mystery-meat-navigation-buttons-some-that-don't-even-look-like-buttons and none of these problems of discoverability would exist.