Concert systems in stadia should always be configured in mono. Same goes for most sports arenas. You don’t want one-half of the audience to hear the rack toms, but not the floor toms, and vice versa for the other side of the room. The high-priced seats in the center may be happy because they’re hearing the stereo mix, but the bulk of the seating capacity will be on-axis with only the L, or the R loudspeaker clusters, and will miss half of the stereo information. As I previously stated this might make the mix engineer happy, who’s usually located directly in the center of the room, but it won’t provide the largest part of the audience with an optimal experience.
It’s always more difficult, and takes much longer, to integrate, tune and align a L,C,R system configuration than a single central cluster, or even a L&R only system. In one-off situations where it’s hurry-up in and out, such as under touring conditions, there may not be enough time available to carefully tune the three main sources (L,C&R) so that that they work synergistically with each other. One solution that touring companies have used in this regard is to relegate vocals-only to the wide-format central cluster and put instruments into the L&R loudspeaker arrays. This can work very well, as it also eliminates intermodulation distortion between vocals and instruments. Prior to the advent of line arrays, some leading sound rental companies commonly used this technique to insure that the vocals were as intelligible as possible, while still having the freedom to pan stereo feeds. Overall, it means using more loudspeakers, but the results can be sonically stunning. These days however, with the addiction to line array usage for everything from a small club gig to a large stadium, that practice has mostly fallen by the wayside because no concert promoter (or system installer) wants a long, tall line array to be blocking the sightlines in the middle of the stage. Maybe it’s time to consider the use of a horizontal array for the center cluster, and line arrays for L&R?