I use the energizer charger with rechargeable batteries and it works just fine...they just don't last as long as Alkaline or Lithium batteries do (one charge...maybe one plus a bit of a charge).

That being said, in regards to emergency use, don't store batteries in the charger (this is what Energizer recommends). I'm convinced that the circuitry leaks current out of the batteries while it's not charging. Also, if you do use rechargeables, buy low self-discharge batteries like Enveloop or Rayovac Hybrids. I carry Hybrids in my charger and cap the ends of the batteries with a little bit of electrical tape. I also top up the batteries every once in a while...low self discharge isn't NO self discharge.

These chargers work on the principle of a 'charge pump' or DC to DC converter. They take a small voltage and turn it into a large voltage all within a tiny IC. This is how 2 1.5V batteries can output 5V DC. Using 2 1.2V batteries will still produce 5V, but the strain on the batteries is higher due to the increased difference in voltage. A weak rechargeable battery won't last very long.

I've tried them with all sorts of batteries and the Lithum AAs by far last longer than anything. I still use the Rayovac Rechargeable batteries pretty much exclusively with the charger however.