1) How often do you use the PURGE command in a drawing?
As often as necessary and always before final saving to network.
2) Do you purge specific things?
Usually All, occasionally individual layers, blocks or styles.
3) If you purge during the drawing process, how do you restore the 'standard' stuff you may need later?
When I purge during the drawing process, I shouldn't need it, if so I use Design Center or recreate it.
4) Is there anything you'd like to purge that you can't?
No, I can get rid of most things, one way or another.