The triviality of Yo is obvious, it's not an amazing feat of engineering, but it is an important app. It shows that millions of people would much rather use an app that takes two "touches" to let someone know they would like to have their attention when they are available. It's a data point on what people are looking for. Many tiny data points can lead to big conclusions and inform design decisions far down that road. Finding out that users still want things that reduce the friction of communication was worth having built Yo.