Seth Godin's blog is an excellent illustration of why it is so important to have a good introduction to your speech:
I especially like the way he likens a speech to a gift. You wouldn't apologize for offering a gift to someone. Why apologize for speaking, unless you really have nothing to say? And if you have nothing to say... why are you there?
About brevity: he mentions that he saw this behaviour at a gala. In galas, people usually have a drink or two. Once that happens, it is much tougher to hold the crowd's attention.
If you speak for more than a couple of minutes, gala attendees will tend to lean over to their neighbours and whisper: "This is kinda long, don't you think." A response will follow: "Yeah, really. I was at this gala honouring Such N. Such the other day and..."
"You were there? So was I, how come we never met? How did you find it?"
"Well, let me tell you..."
"Shhh..... I'm trying to listen!"
This tends to be repeated over and over again until eventually, speakers are drowned in the... hush of the crowd.
The best remedy? Get to the point immediately, conclude quickly, go back and have some fun!