Advantages: You can mix the exact color and tone you want before adding it to your painting. Depending on how much you mix, you'll have some for later on in the painting.
2. Incomplete Mixing on a Palette
Advantages: Variation in color adds to the visual interest. Easier and faster than painting small bits of color separately.
Disadvantages: Results a bit unpredictable. Large bits of unmixed color can spread out and ruin effect.
Advantages: The color is right where you want to use it in your painting, without needing to be transferred from your palette.
Disadvantages: It's riskier, as you could create a mess on your painting by mixing the wrong color, mixing too much, or inadvertently mixing into a bit of the painting you hadn't intended.
Advantages: Layers of paint add depth and variation to the colors in your painting. Variations in color engages a viewer's attention by making you look a little longer.
Disadvantages: You have to wait for the first layer of paint to dry, otherwise your scumbled layer will mix with it rather than sit on top. Too much scumbling and the technique becomes too prominent in the painting, distracting the viewer from the overall composition.
Glazing does take patience. For starters, you have to be sure a layer of paint is completely dry before you paint over it, otherwise you'll have a physical mix rather than a glaze. You also need to know which of your paint colors are transparent (the paint tube label should tell you), and understand the basics of color mixing to be able to predict what color you're going to get.
Advantages: Produces deep, rich colors.
Disadvantages: Requires patience to wait for each layer to dry, and to acquire a good working knowledge of your colors (which are transparent colors and what the result will be when you glaze a particular color on top of another).
Pointillist painters such as Seurat were masters at this, working with uniform dots of color. But even if you don't go to the extremes of having ever bit of color the same shape and size, you can use optical mixing to make your colors seem to dance, add a vibrancy.
Advantages: Colors engage the viewer's mind, as the eye spends time interpreting what is being seen. Colors aren't static.
Disadvantages: Takes patience to work with small mark-making, to not blend colors, and learn how colors interact when placed alongside one another.