it's to get the best experience over all. I have a gearvr that runs at 60fps and most of the time it's really good althogh some experiences can make me want to play something else.
My DK2 at 75fps works really good when the games/experinces can maintain that. Otherwise it can be vomit inducing. That's usually only in things that are hacked to run VR through an addon. Many games that never planned to run in VR are not a good time.
They are pushing for 90FPS and have a tech in the SDK called time warp which can help with minor drops below that by slip streaming fake frames.
Over all I'm pretty excited about both the new Steam VR Vive and the CV1. I don't have a PS4 but if i did I'd be excited about Morpheus as well.