You don't need to *see* it, but your brain can notice it. It adds to your impressions about the app, it won't look right. Just like a slightly detuned string in a guitar can make a chord feel wrong. If you play that to people, they won't be able to tell you it's a detuned string, but they perceive "something" different.

Now, the Google example is totally exaggerated. They did it because it's pretty easy to setup an automated A/B test and collect results in a couple hours/minutes. It didn't require weeks of effort, probably a couple hours at most, and it doesn't represent the usual design process at all.