Lighter cues have a couple of advantages that have not been touched on by other posters.
First of all, in a long session of play, with equally matched players the player who gets tired first will lose. For those who have played for 24 hrs or longer, the weight of the cue can affect how you stroke the ball, the heavier the cue, the more the weight will effect your stroke.
Second of all, the heavier the cue, the less effect a player will get when breaking or applying english. This effect is due to a cues weight having a overall deadening effect on the imparted action that results when the cue ball strikes a rack of balls or a individual ball.
Most 3-cushion Billiards players are aware of this effect, and due to the table size and the additional weight and size of billiard balls Billiard players use lighter cues. Billiard players do not prefer to use cues heaver than 18 oz for this reason. This is where I first saw this concept work, however I am uncertain how light you can go before this effect is nulled. I use a 17.8 oz cue myself daily, I have built and shot with cues as light as 15 oz and the effect was not intensified with the reduction in weight, so I am not certain that going below 15 oz will have any effect other than feeling comfortable to some people.
If anyone questions what I have said here, please test my idea's. Try breaking / shooting with differently weighted cues, and you will see the effects that I have outlined above.
Exactly why it works, I do not completely understand, however I am certain that anyone who tests this concept in a subjective manner will see the results and then maybe someone can explain further why this works.
Manwon