I could only hinder a guess at whether or not it keeps group members sane. Personally, I know that handling an entire forum for a roleplay can drive one mad. Though, being a member of a roleplaying site makes me as a member more active. It gives me a sense of purpose to stick around and make friends.
For me, I like a site that looks like it was taken care of when it was being made and developed. Not some random, pre-made, stuff slapped together with clipart everywhere, I want a site to look as if it was an A+ student's research project. When it all comes down to it, one must ask questions to determine whether or not the site reflects the roleplay. For example, is the roleplay an apocalyptic hierarchy? If so, is the website rugged looking, like a journal that is being continuously written in everyday; recording the sanity of its members. Not sure if this is coming from the imaginative and perfectionist side, but I hope you get the jist of what I am trying to conclude.
Laymen terms: yes, I think it is important and I will 5/10 want to join a roleplay if it includes a DETAILED and INTELLIGENT-ORIENTED site.