Three big problems with defence for me:
1) A lot of people simply don't realise just how much the military does. The RN's the Service I know best, so a couple of examples from there - Caribbean patrol (regularly doing disaster relief ops during hurricane season), anti-piracy patrols (90% of our imports are sea-borne), Fisheries protection. Also a little known fact - counting the Marines, as they're part of the Naval Service, the RN has had the largest service presence in Afghanistan more than once.
2) There's an assumption that the forces scale linearly, so you can do the same things with a smaller force, but not on as great a scale. Unfortunately that's not the case, any further reduction in spending will lead to loss of capabilities, such as amphib landings. Once lost, those capabilities cannot be easily regenerated if needed - one of the reasons it was such a dumb reason to scrap the Harriers when we did (we have a few pilots with the USN operating F18s, but we're haemorrhaging experience and F18 experience is not particularly relevant).
3) The Service ethos is to get things done, and will stretch and stretch and stretch. That's good, to a point, but when you rely on things at full stretch they will inevitably go bang sooner or later, and often spectacularly...