Drawing using semi-transparent shading, lines, and other objects is processor intensive. Expect semitransparent objects to take approximately twice as long to draw as non-transparent (opaque) objects.
On the other hand, using transparency can actually speed workflow. For example, without transparency it may be necessary to draw the contour layer first and then a map layer on top in order to prevent the contours from obscuring the map layer. When zooming and panning with opaque objects, one must wait until the contours are drawn and the map layer update before determining whether or not the desired view is attained. However, with the semi-transparent palettes, the map layer can be drawn first and then the contours on top without completely obscuring the map layer, making the desired view more effective with zooming and panning operations much more efficient too.
Semitransparent palettes can also be used create some more interesting ways to represent data. For example, suppose that one wants to display a dataset representing turbidity or humidity or cloud cover. All the colors of the palette could be set to be the same but with a varied transparency amounts to give the desired effect of a scale that goes from clear to hazy.