As far as I see it, there are only two issues that needed to be resolved:
1. Use of personal equipment when used for the company needs to be realized and properly accounted for.
2. Work on the OP's own time doing post work needs to be either paid out properly or somehow compensated for.
All this dribble about job descriptions and not hired for this position is just that dribble. Would you really rather sit everyday, day in and day out doing only what you were hired for till you kick the bucket, or do you want to expand your worth to the company, be flexible in what you can do and possibly grow within the company.
If you want to live inncubicle 23 forever, make it difficult for the company to see your expanded value for the company. If you are a go getter, jump at opportunities as they present themselves...you'd be amazed where open doors will lead...right past cubicle 23.