What's the difference between cheap pencils and crayons for coloring? I heard that colored pencils use wax (or clay) as a pigment vehicle, and crayons (wax pastels) use wax as pigment vehicle.

I found recently that crayons are mind blowingly cheap (96 crayola crayons for $6), whereas colored pencils tend to go for at least $2 for a pack of 24.

I am starting to think I should just switch to crayons for when I need to get some color down, especially if I intend to take photographs of them rather than frame them.

I tried searching for what the differences between crayons and colored pencils were, but I couldn't find anything. 

My question are:


Are  there any drawbacks to blending crayons with pencils (such as crayola/prismacolor scholar)? 
Are  there any drawbacks to using crayons over pencils?
I couldn't find any lightfastness information, and can't think of any reason for them to behave any differently than a typical pencil other than not having wood, and being softer.
Quality will be much higher with pencils. The softness of crayons dramatically reduces your precision when drawing, and they have a tendency to "skip" - I guess because of the higher amounts of wax involved. This is why crayon drawings rarely have a uniform color field, and don't blend well.

You will use up your crayons much faster than pencils as well, because the soft wax smears much faster. As you mentioned, the wood of the pencils also lends durability. I've never had much luck mixing the two unless I want to see the different marks - the materials are different enough to stand out next to one another.