Applications do not integrate. This is trule. This is bad, because we actually have a common standard for this. We just have different mimetypes. Competition is a good thing.
People like to show off, basically.
When they heard about the nautilus shell's previews, they just had to go off and implement it in KDE, too. This benefits users. These questions are pretty academic, because we're here now. One big plus point is that the choice benefits end-users. It benefits developers, too.