The main benefit to having an ETF is that it legitimizes Bitcoin for the mainstream investors. I knows tons of people who simply do not trust Bitcoin because there is nothing backing it but investor confidence and mathematics :-D I tell them that is why they SHOULD trust it, not the manipulated stock markets and precious metals markets which are controlled by the useless dollar.
In order for the blue-hair investors to jump on board there needs to be wider adoption and the ETF would bring that to the masses.