How Douglas Adams rounded out his ending to the series:

...maybe nobody noticed it, but: Not only is there a Moral (of all things) to Mostly Harmles (=H-H.G.G.-'V'), but a multiple duality at the end.

The Moral: those who are unsure of their place in the scheme of things are dangerous. The dualities:

   A (1): Random is bewildered and desperate  (2) the Grebulons on 
 Rupert are unsure of where they stand in regard to
 everything. (And both commit destructive acts.)   
  
   B (1): A spacecraft lands in Central Park    (2) Another one lands
 shortly after (-headline: "'This One's Pink'".)   
  
   C (1): On  entering Stavro Mueller's 'Beta', Ford Prefect says (of
 Arthur), "It's OK, he's with me."    (2) Arthur shortly thereafter says "If
 I'm here, we're safe."   

   D(1) & (2): Arthur hears his name shouted 'in stereo'; there are
 two Trillians present.
   E(2): The Earth is destroyed, with Prostetnic Vogon Jeltz looking
 on.

- Just as at the beginning of the series, (1) so at the end.

And there is reference to the right brackets (that close out a section of code, by canceling the effect of a previous left bracket) in programming; and the bird had resolved itself out of existence too.

Please Note: all morals expressed here are what I've extrapolated from Douglas Adams' writing. Not saying I believe 'em; Need a safeguard, what with Mark Twain's dedicatory statement to 'Huckleberry Finn'.
-R.H.