What good is a Center Channel Speaker?

 The question in a thread on Audiogon recently came up which I thought would make a better blog post than an answer in the thread:

2. How much of a difference does a center channel make for video?
   

My first real jobs were centered around motion picture sound systems, and I recently built my second center channel speaker. 

If all you are asking is whether you should go from 2.0 to 3.0 I'd say no, with some caveats.  Originally center speakers in theaters were needed because of the wide variety of listening positions.  Customers could be very close to the screen but at extreme left or right angles.   Among the goals of Dolby Surround (later called ProLogic and then again Dolby Surround for homes) was making sure the dialogue and center effects stayed in the center, in addition to adding surround effects tracks.

Along the same lines of thinking, I recommend a center as important when you have a variety of seating positions (which I do) and your screen is particularly large / speakers are very far apart relative to the screen.