Having qualified rather objective ideas about the Mental Universe and discussed arguments against these ideas I'd like to move into why the Mental Universe is important.
Inherent in the concept of the Mental Universe is that it is subjective. Without our cognitive ability the Universe can said to be nonexistent. To prove otherwise is virtually impossible. The importance of this concept allows one to appreciate the value of the individual. One's existence is essential to the Universe and thus comes the ideas of universal rights and freedoms (all inlaid with respective responsibilities, but that is the subject of another discussion). To understand the worth of the indivual to the universe is to understand life within the four dimensions of the universe.
One the best philosophies of our time to recognise the value of life is Utilitarianism. It promotes the value of life and regards the quality of life as the ultimate goal. One of its modern day proponents and probably its most well known is Peter Singer. Singer has taken this pre-Victorian thinking developed by Jeremy Bentham and John Stuart Mill to encompass a gobal society which takes into consideration all forms of life. Often seen as an extremist Singer condones the the actions of a hedge funds billionaire as long as his earnings are used to fund works of charity for the poor. By the same token he may disdain the raising of thouands of dollars to cure a child of cancer while feeling the same amount would go to better use to buy mosquito nets in poor African villages to save the lives of hundreds from malaria. Logical thinking: no doubt, but the accusations of extremism are no difficult to justify. Of course I am cherry picking some of Singer's more controversial arguments but his reasoning does lead to the aforementioned conclusions.
The concept of the Mental Universe, however, gives the individual the reasoning to avoid such extremes as represented by Singer.