In the spirit of flat design, I am not a big user of gradients in my presentations. It is one of those features: the fact that PowerPoint/Keynote supports them, does not mean you have to use them. Some observations.
Not all gradients work. A background gradient that goes from white to a touch of grey as your PowerPoint canvas often looks “dirty” on the presentation screen. Especially on antique VGA meeting room projectors. The inverse (pitch black to a dark grey) can actually look good. There is another challenge though with a gradient slide background: it is harder to work with shapes and images that have a non-gradient background that is close to the canvas color.
Watch out with gradients that run between clashing colours. If the colours do not go well together (for example green and red) then the resulting gradient is probably not going to be good either. Complex gradients can work though, have a look at the book cover of “Pitch it!” on the blog cover page. You could construct a nice gradient with reds, oranges, blues, and purples.
There is one area where I often use gradients: visualising transitions from one state to another. Even if the colour clash, I would still add that colour transition on a big horizontal arrow.
But still, we have to admit modern display technology falls short in places where ancient artists thrived...