Speaker Delays and Wide Speakers

It is common practice to use multiple layers of speakers when dealing with larger spaces. There will often be a main speaker array, and the several fill (or "delay") speakers. When using these delay speakers, it is standard to add a time delay to the audio feed leading to those speakers to account for the propagation of sound throughout the space and attempt to minimize any perceptible echo and phase interference. What happens though when your speakers are far to the outsides of your audience