sparring will ALWAYS strengthen the core attributes of timing, accuracy, and defense
Not sure of how good your knowledge is about WC, but we use chi sao to work these attributes.
Sparring for WC is pretty much just playing around for us. Take a look at this: http://www.metacafe.com/watch/617621/wing_chun_sparring/
It appears silly and soft and kind of flamboyant, if you ask me. Almost pointless, other then the interaction with the training partner. The subject to the video states that they are sparring, to me it kinda looks like a free roaming chi sao session. Either way, they're just moving around and slapping each other. There is no energy battle of structure or definite move, as if it were a real fight. A WCer strives to end the fight with as few moves as possible. This sparring session here is just a waste of energy and is really one guy, the bigger one, showing how he can use intimidation to over power another student during training Non of the moves being thrown actually have a purpose other then point A to point B and "hey dude, I just got in on you"
The only way to truly work on WC fighting is to actually go out and use it.