There are issues with making the pen rechargeable. Things like either the pen would need to be made bigger or the internal battery much smaller (in order to accommodate the charging module and port), price would increase by a fair amount, battery life would be fairly limited, to charge you need to use your only USB port (which means you wouldn't be able to use an external drive or other device while the pen is charging).
This is not even mentioning the fact that a USB cable for charging the pen would take up far more space than even a couple of replacement batteries. While it is possible that the cable could be used for multiple devices, surely as a Road Warrior you won't even want to take the cable with you. I know I don't.
While it is a nice idea, in theory, to have a rechargeable pen, practically I can see it causing more issues. In fact I imagine there would be far more complaints about how short the battery life of the pen is and how it has to use the only USB port than they will receive from the fact that the single AAAA battery has to be replaced twice a year.