
Flash can be useful in some situations but not all. some pictures often look better taken in the dark with little to no excess light. the pros and cons of flash are as followed:
Pros:
A) flash will elimate all blur in the picture.
B) flash will also brighten and show all aspects of the subject.
Cons:
A) Flash dulls the picture by elimating other lights and colors
B) It can also add red eye and show defects in skin.
C) Flash causes the background to extremely darken or be pitch black.
D) It also causes glare from any glass that may end up in the shot.
Flash can be a useful tool in the midday hours or during twilight hours. it can also be useful when the subject appears TOO dark to even see and metering and ISO fails.
Flash is NOT useful when one wants to see the background or has colored lights showing. flash also takes away shadows from everything around the subject.

The Picture above demonstrates how using flash would be a poor choice. Although the subject is the sitting sphere, the colors around the ball add on to the pictures. flash would have elimated the colors around the ball.