Well, many are useless for evaluating exposure.
The photo looks great (bright) on the LCD monitor, but when you download it to a computer, you realize that the image is actually underexposed.
Many LCD monitors are designed to produce a very bright view of an image in Playback mode. That's great in theory, but not if the depiction is inaccurate.
(Setting LCD Brightness to -1 is possible with some cameras, a good idea if the default setting makes images appear brighter than they are.)
A histogram feature -- or warning blinks for excessively bright highlight areas -- is a valuable feature. But not many digicams offer those.
Of course, most people cannot interpret a histogram. That makes the warning blinks more useful. Parts of the image flash to indicate excessive brightness: loss of detail. Some cameras also flash a warning over excessively dark areas of the image.
All digicams should included a feature of this type. It's not a perfect solution, but useful.