render :action is not better than redirect_to, it's just different. render makes rails use a different template, redirect_to makes the browser resubmit a request to the specified url.
Both have their place, although it is true that there are cases where one is preferable than they other.
One of the cases where you do want to use render is when validation of some user submitted form has failed (in particular if you did redirect_to :action => :index then you'd lose all the stuff the user submitted)
Fred