I got a chance to sit in while a local Bose installer gave a demo at our church. It was very enlightening. They brought a test rig that allowed them to place two columns (one on top of the other) in various locations. They then ran various test material into the system while we walked around the church and listened.
The resulting clarity was astounding compared to our present sound system. This church is very acoustically problematic and the line array systems are the obvious solution. The key is that they keep the sound from going into the upper portions of the facility, which add a lot of unwanted reflected sound. That 2-column tall test rig was an immediate improvement over our present system.
They did explain that, for a room the size of our church, they would go with 3 columns stacked on each side. The key factor here is how far you want to project the sound. The more drivers you stack vertically, the better the sound carries to the back.
While a number of L1s mounted on a back wall might work in smaller rooms, bigger rooms need more penetration and, therefor, taller line arrays.
The downside to the professional installation is simple: cost. This would have been a very expensive installation and our church hasn't been able to justify the expense with the present faltering economy. I really wish they could find the funds, though, because the Bose install would be such a massive upgrade over what we have now.