It's not rare at all. A double takes up twice as much memory as a float and memory bandwidth is a precious commodity these days. If you have a lot of numbers (e.g. in 3D models or similar) and don't need the extra precision, using floats is an obvious choice.

Most video games use floats. This is because doubles will be emulated in software on some platforms, thus making them incredibly slow. The Unreal Engine, for example uses floats extensively.