Oh, I see what you are getting at.
Using two resistors to drop 12V to 5V works, but it is very dependent on the ratio of current flowing through the load (in your picture D1) vs the current being 'wasted' in R2.
So if R1 is 7 ohms, and R2 is 5 ohms, 1A will be flowing through the divider and with no load connected you will have 5V.
If you draw of any appreciable amount of current to power an LED or Arduino or something it will cause the voltage at that point to drop. As a rough rule of thumb for this case, drawing off 10% of the current (100mA in this case) will cause the voltage to drop by 10%.
If the load current jumps around (e.g. an Arduino blinking an LED) then the voltage will also jump around. If you want a stable 5V, and don't want to waste a lot of current, then you need a voltage regulator.