The fact is that we don't know what we might need. This Crimea business has shown the West that, nobody saw it coming, seemingly, and it could be read into that Mr Putin seized the opportunity of Western defence cuts to bring about not only a geographical victory, but a boost to his world standing too. It could almost be argued that in the real world Russia is the ONLY superpower, in the sense that they have all the prerequisites, and are prepared to exert that power to achieve their aims.
You can't tell what we might need any of the military capabilities for, current or historical, and we can only hope that we don't find ourselves wanting in a time of need. In a world where the "flash to bang" time is much shorter than the development time of war machines, we need to have all of our toys on the start line before it starts.
In an ideal world defence spending would be calculated on the basis that anything can happen, especially the unexpected, and would consist of a proportionate chunk of GDP relative to what you were willing to pay for survival, not a particular perceived threat.