Unlike meds for something non-life threatening like, say, a headache, an Epi-pen could readily prevent a person's death. In such a risk/benefit equation, I personally wouldn't play around by knowingly carrying an expired Epi-pen although I know they're not cheap. I'm not saying that it's useless the day after the expiration date. The point is that there's no way for you or any of us to know how much or how fast the potency of any particular Epi-pen degrades once it's past the expiration.

Keep the expired Advil or canned goods if you'd like, but don't scrimp on the Epi-pen if you're going to go through the trouble of carrying one as potentially lifesaving measure.