Steve Lawrence discusses the importance of online research papers in the paper Online or Invisible?:
The results are dramatic. There is a clear correlation between the number of times an article is cited, and the probability that the article is online. More highly cited articles, and more recent articles, are significantly more likely to be online.
Free online availability of scientific literature offers substantial benefits to science and society. To maximize impact, minimize redundancy, and speed scientific progress, author and publishers should aim to make research easy to access.
This is something I see very clearly both in my own research and among my colleagues. It is very rare to not find interesting papers freely available online, and it is obviously easier to use (and cite) material we have easy access to.