I was just perusing the Wiki and going over the formulas for item value. From what I can see, the default value of any decoration is 10, prior to material multipliers and quality multipliers. This value for the decoration is then added to the value of the item it decorates; so, for example, you might have a masterwork (×12) steel (×30) wheelbarrow (50), worth 18,000☼, with masterwork (×12) gold (×30) studs (10), worth 3,600☼, for a resulting ☼«☼steel wheel barrow☼»☼ worth 21,600☼. That looks like a good deal, but for the same materials we could instead build two masterwork quality wheel barrows, one gold and one steel, each worth 18,000☼; thereby, we would gain 36,000☼ from our two bars of material rather than merely 21,600. I have looked through a few combinations of materials, and it seems like there is generally something more profitable than decorations to use for a given material. So why would you make them, except during strange moods? Not only do you get more total wealth from making items rather than decorations but also you get more individual stuff: more bins to store things in, more clothes for dwarfs to wear, more weapons to hit things with, or whatever.
If there are some examples I have overlooked of materials for which using them as decorations (base item value 10) is more profitable than to use them to create individual items, then please let me know!
Thanks in advance.