You don't have to get an AC charger so long as you don't mind waiting 3-4 hours for the ereader to charge on the USB as opposed to the 2 hours it takes on AC charger.
I wasn't aware that you could read it while plugged into the AC so that's useful although I'm now used to going away and doing something else while waiting
To be honest I could have managed without the AC plug as I'm only charging it up once every 1-2 weeks.