At our club race we've been OK with people asking to swapping numbers. In pactice it's probably averaged out at around 1% - so for us 4 or 5 entries. Simply get them to fill out an entry form & swap them - no big deal.
However if that multiplies up at a big event with thousands of runners then it could start to become an issue. That could be 3-400 hundred people at the biggest event; GNR, VLM etc.
Plus there's no great incentive for the RD to do it. He's legally responsible for the event. When there's a problem it's him the authorities turn to for info, whatever may or may not be written on the back of the race number.
We've only had one occassion when a runner collapsed and died on the course. It turned out to be a runner not in the race who just happened to be passing, but it was a horrible time for the RD 'til things were clarified.