I think the way this is expressed will sum things up quite well on this since often I hear that one monitor is simply better then another simply by looking at that alone. After a certain point however it generally wouldn't make any real difference.
"Contrast Ratio for Televisions & Monitors
Contrast Ratio is a specification given with most good televisions and monitors. It refers to the device's ability to reproduce different levels of brightness.
A higher contrast ratio is more desirable — 500:1 is quite good.
Unfortunately the contrast ratio specification given by TV manufacturers has become somewhat abused. There is a limit beyond which this specification loses significance, as further improvements aren't noticeable in the real world. Do not be too concerned with figures higher than 2000:1, and figures over 10,000:1 don't have much relevance at all." Contrast Ratio for Televisions and Monitors