Sometimes there is little time to do your pitch. You meet an investor in the corridor, you got a slot at a pitch competition. Seth Godin said the other day that no one has ever bought anything in an elevator: in other words a very short elevator pitch consisting of 2 sentences with hollow buzz words is not going to excite an investor.
Instead, you need to add more specifics to intrigue the investor to invite you to another occasion where there is more time to discuss your idea.
But sometimes, elevator pitches become less effective when you take too much time. You start adding details, provide facts, that take the energy out of your presentation, that require time to close all all the plot lines. There is a dip between the perfect short pitch, and the full-length 25 minute story. Do not get stopped in the middle, it is better to keep it short.