Those who have seen my posts know I'm not much a fan of absolute statements. This is definitely one of those cases. While on the whole, this is definitely the ideal, to say that you MUST have a matched front 3 speakers is being rather blind to the compromises we often have to make in the process of integrating a home theater into a home.
The first thing I will note, is that when heavy compromises don't have to be made, I will fully agree with the above sentiments that the LCR should be sonically matched speakers. Where I question this practice is sometimes where budgetary, cosmetic, and space constraints come into play. Let me give a simple example using the NHT product line as it is reasonably well known. Let's say that our theater in question is in a family room where cosmetics are very important to the owner. Due to space and cosmetics the largest speakers that can fit for right and left duty are a pair of Super Ones. Now, due to the size of the TV, there is ample room for a center channel which would be easy to conceal. If the owner is interested in watching mostly video sources, and want's the best quality he can fit into this location, I say we would be doing the owners a disservice by using a SuperCenter just because it "matched." Instead I would install the VC-3 center channel, which is a robust, 3 way center channel from the VT-3 system. These, paired with a large reciever or separates, and a healthy subwoofer, would give SIGNIFICANTLY better performance than the Super Series combination. Is it ideal? No way, but it maximizes the owners enjoyment while being acceptable to the limitations of the application. Of course there are many other, and better examples, but the point is still made. In some cases, stubborn interior designers had only allowed for in-wall speakers for the right and left (small ones at that) and the possible "matching" center channels would have been quite a compromise for what the owner of the home was looking for and could afford. Maximizing the performance in areas of most significance is just good value engineering of a system. Of course people should be aware that their system is less than optimal, but that becomes their decision as to whether or not the required modifications can be justified. Fortunately there are no "home theater police" to arrest those committing such sins http://www.avsforum.com/ubb/wink.gif