The intellectual part of me understands that it would be better without showing the Xenomorphs in IT CAME FROM OUTER SPACE (and The Demon in CURSE OF THE DEMON). 

However, the eternal monster kid in me LOVES that we see the monsters in both films.

The only film I can think of off the top of my head that I really love the way they handle *not* showing the "monster(s)"is THE ABOMINABLE SNOWMAN FROM THE HIMALAYAS.