Or are you forcing people to scrub over every element on the screen to "discover" instead of it being visible to begin with? That's annoying as hell and people are probably missing things because you chose to hide them by default.
The human eye is very sensitive to motion. It can be a fun thing to add to loading screens (the gusto pig, for example), but icons are designed to live on a page for a long time and only take our attention when we are actively looking for them. Motion is not good for this purpose.
I can see animations like these working for when a user interacts with something, as an alternative to just highlighting or doing a material style ripple. The problem is that most of the time I'd want that, I'd also want to transition between two states, which—unless I'm missing it—isn't something this offers.
Also, I can't agree with "Let's not just do this because we can." enough.
There isn't even an argument that doodads like these can placed on websites in an attempt to monetize users, like ads, where all the debate about sustainable sources of revenue and the ethics of adblocking come into play. They serve no material purpose at all except to be distracting.
Animations of any kind, even the ones that are finite in lifespan or simple page transitions, already annoy me greatly. If we lose this front, where random businesses start adding things like this only because the web platform has sufficiently advanced technologically to allow it, and a majority of users become desensitized into believing it is "cool" and shifting the the baseline of the web such that this majority now expects and might even want this kind of behavior, it would surely become another unnecessary annoyance. I feel like this has already happened with Metro/Flat design and the death of skeuomorphism (more or less), and also the overuse of animated GIFs in blogs.
But there's nothing making it outright illegal, so I'm not sure what would stop them from doing so at this point. We can't exactly go back to Web 1.0 (nor should we) - that kind of design is opt-in now.
After all, we only got rid of animated GIFs not too long ago.
[under-construction.gif]
An animated green check mark when showing a popup confirming that an action has succeeded, an animated red cross when showing an error, wheels of a car icon rolling when hovering over the car option in the menu, those kinds of animations can all aid design by showing the user that the site or application is responding to user feedback.
It makes the user flow just a tiny bit more interactive and polished without being too distracted. After all, if there's a popup, the intention is to direct the attention of the viewer to that spot already. Same with a menu icon.
That said, the animations shouldn't be too fast. Some examples, like the moving train, are way too fast to be comfortable to use in my opinion. Proper pacing is important.
This does not only apply to vector icons, by the way. Most highly polished icon sets will have an icon for different sizes. For example, in Windows, zoom the desktop icons in/out and compare.