A tablet pc really is a glorified displaying device which can do bit more than just display and nothing else. Wouldn't spend on such a device for editing photos.
No I would never edit photos on a tab. I need my portable option for reviewing images live in the studio since the camera display is to small, writing and editing documents (research papers) and take notes on lectures.
Anywhere you can read a book. There are more e-ink devices with built in lights now though.
I like to read in the dark so a lit display is a must for me.