They have a good point!
Though, I'm not sure of the value of running multiple different AV programs. Not for their reasons, but simply performance and incompatibility. AV programs have historicly been very incompatible with each other and even causing problems with other programs: There's good reason that many programs recomend that you disable AV's during installs. Then there is profromance issues, with each program examining file data during reads/writes/opens etc. At some point, the dragg on the I/O subsystem really starts dragging down the performance of the entire machine.