Linda,
 
I'm not sure that this analogy will fly on the 6 o'clock news but when faced
with similar questions from some uninformed individual, I usually compare an
archaeological site to a book (if you like  this approach you should credit the
long forgotten individual who I borrowed it from).  In your example, bulldozing
the site and recovering the whole bottles, the site could be compared to a 300
page novel with 15 illustrations.  Bulldozing the site and saving the bottles
would be like saving the pictures and throwing the text away:  interesting stuff
to be sure, and a story could be told, but look at what would be lost.  The book
analogy works for artifact collecting - ripping pages out of the book, etc.
 
Good luck!