I've been speaking in front of large crowds of all kinds of people (students, faculty, parents, peer-leaders, members of communities and others) for 18 years and to this day I cannot bring myself to script a speech. I don't memorize my "lines" either (one of my intelligences is surely not the capacity to memorize big chunks of information) or spend a lot of time preparing what I'm specifically going to say.
What's my problem? Maybe I'm too insecure about my incapacity for memorization and this stymies me from preparing talks for large crowds. It could be laziness. Whatever the malady, I'm resigned to thinking "big picture" about what I will say when I have to speak formally in front of a throng of people; I let the details follow.
Pastor Mike (the Reverend Michael A. Walrond of the famed First Corinthian Baptist Church in Harlem) does the same thing I do, and he thinks of it like this, "I don't write them [sermons]. For me, the construction takes place in the pulpit. I think about the jazz improvisation. For Miles Davis or Coltrane to do improvisation, they knew the instrument, they knew the chords, they knew the keys. For me, it's the same thing..."
You can engage your audience if you know your stuff and have a passion for what you're talking about. This might be the way to effective speech making without formal preparation.