The hipster moleskine works best at a certain volume, so I keep a lot of spare cards inside. This is also the idea behind Dave Gray's hack of using this as a notebook.
One thing I've noticed, in the years I've been using index cards, is that when you feel like you're low on cards, you don't use them as much, which defeats the purpose. You want overabundance, and instant access. Which is why there are blanks clipped to the back.