The scenario is as follows, a user accidentally enters a value from a pop up with a non editable value list. Then they want to edit the field and set it back to void. Well, they can't unless you script a button to do so, which to me seems clumsier than having a " " in your value list and maybe a - before or after, separators being great for such staff. 

 

As an aside, in find mode you again can't use the field to search, as far as I can tell, unless again you put in a button or some pop up. Another way I would go about that would be to have to fields overlaid one visible in browse and the other in find mode, that's my approach at least. 

[…]  Well, they can't unless […] you script a button to do so, which to me seems clumsier than having a " " in your value list and maybe a - before or after, separators being great for such staff. 

 

… they hit the Delete/Backspace key.

my bad