There's no reason for the average investor to trade ETFs at all when no load mutual funds exist. You can use vanguard directly or etrade (or a billion other companies) for that.
For non-Vanguard funds, ETFs are generally more tax efficient than mutual funds because they can avoid capital gains distributions in ETFs, but not in mutual funds. Vanguard has a patent on the mutual fund method.
There are minimums investment amounts on many mutual funds include the Vanguard ones which are most popular with personal finance investors. ETFs don't have minimums allowing for more home investor participation, especially from those new to investing or personal finance.