You're right. The number of phones don't determine whether the driver is capable or not. But for each phone call that the driver takes while on the road, your own safety is compromised. The drivers with multiple phones often have to juggle several calls at once. They are maximizing their profit at the expense of safety.
FYI, my wife and infant son took a taxi recently. The driver was juggling multiple calls when he unknowingly ran a red light. Then bam! They were sideswiped by a turning vehicle. We were fortunate in that no one was injured but there's no doubt that it could have been prevented if the driver paid attention to the road rather than his calls.