A program that can run on 1GB but uses 2GB is more wasteful, OS and FS level caching and memory reclamation only work if the memory is available, and a program wasting it takes it from everything else, unused RAM is wasted, but so is RAM being used for no actual function.
Not to say programs cant use large amounts, but they should provide a level of functionality for the amount of memory used, and some programs of late have been more than a bit inefficient, in short, filling the RAM is good, but do make sure its actually being used.