It's the unofficial prerequisite for joining the EU, with few exceptions. And what did Nato gained? They gained access to airspace, military bases, boxed in Russia etc.
IMO the only thing that changes when neighbors of Russia join NATO is the major decrease in the possibility that Russia might invade them and that's what Putin really is crying about. There's no real risk that anyone would try to invade Russia, a country with thousands of nuclear warheads.
