Iron -- even some trees can increase appearance, but should NEVER be used to hopefully "hide" ride imperfections (like using amazing 3D's and attaching them to a crap ride hoping for good marks). It gives the coaster that "finished look" where the SIM can make it look so empty. Now, for RCT1 - RCT3, scenary is a must. But in NL, like I said, it just gives that finished look and almost makes it appear to be more real, IMO.
However I agree ... scores should NEVER be affected by lack of scenary, but when it is there, and done well, I always add to the score.
See this for how I rate (loosely done of course with no real values added):
http://www.freewebs.com/tconwell/rating.htm