I have set up, started a recording and walked all over the room and can't hear any noise cancellation. I know that in theory it should be there, but in reality if I can't hear it, I guess it isn't noticeable.
Although some sounds are panned center, including voice, many are panned left or right to varying degrees and therefore shouldn't be affected unless they happen to share an exact same wave length.
Could it be that noise cancellation is more noticeable on conventional speakers than it is on the line arrays?
I do recall having two passive speakers in my club, where it was necessary to move one of them a little because of this factor.
If I were regularly playing for more dancers that want to really feel the thump, then conventional speakers might be more in order, particularly, as Fran noted, if you wanted the sound level to fall off.
And, despite what the manufacturers say, there IS some falloff. It is not as loud at the end of the room as it is ten feet away. However the sound throughout the room remains at a listenable level much better with line array.