I do not mean to turn you off entirely to missiles.
Each person has their own play style and you may find missiles very useful. I will use them when "role playing" a particular country for example, but in general game play they just do not hold as much importance to me as they did when I first started playing.
As best I can recall, all true arty units will indeed do damage with or without missiles. This obviously does not include the launch trucks/trailers some countries have. (think Scud launcher)
EDITED to add: Have a look at this viewtopic.php?f=46&t=12348
. Ruges has made an excellent "guide/overview". Should still be accurate enough. Do some playing around and see what works for you, missile wise. Good luck.