One possible solution might be to set a preference to "keep at most X 
drafts".   Not sure how hard that would be to implement or anything, 
but does sound somewhat useful to me...

Both has its use. It depends on whether you use the drafts feature for 
templating or really for drafts. Once you use it for both you would 
run into problems if saving drafts is overwriting the original draft. 
We already do this btw. when auto-saving drafts in DIMP.

So far imp always adds a new email to the drafts folder when you click 
on "save draft", even if you continue an email. I guess it would be 
nice if previously saved drafts get overwritten.