I actually wish that more races offered bib transfers. If they did, a lot of the issues about running under someone else's bib would go away.
With the amount of money it costs to enter races these days (hundreds of dollars sometimes) and how fast they fill up, it is sort of ridiculous that you can't transfer your bib legitimately in most races. Almost everything else in the world can be transferred. You can transfer (aka give them away, or sell them) tickets to a concert or a sporting event if you can't use them. A bib if like your ticket to the race. The fact that your bib is tied to your name/age/sex, is more of a technical hurdle that needs to be overcome in transferring the bib, than a reason to not allow transfers.
I believe it is because there is no financial incentive for race directors to allow bib transfers, that it isn't done more often. Taking Disney, for example. They have already received your money for the race months ahead of time. Whether you run or not, they make the same amount of money from the race. In fact, if you don't run the race, they actually expend fewer resources on you and it saves them money. Most race directors count on a certain number of no-shows in races (for marathons it can be as high as 10% or more) and buy their supplies accordingly.
So while I agree that running under someone else's bib is "technically" against the rules, I don't have a problem with it since there is no other alternative.
Sure there is the downside of the results being skewed, but that is pretty minor in my opinion. I, for one, care much more about my personal time and racing against myself than my placement among my fellow competitors. Also most of your overall placement and age group placement has to do with what race you run. For example, this thanksgiving I finished in the top 5% in a large 5K turkey trot that was run for charity. That wasn't because I ran a great race, it was because this race had lots of people walking, or pushing strollers or running with their dogs or just out to have a fun time, rather than race super fast. I could easily go out next week and run a faster time in a small local race and not even break the top 30%. My overall placement in a race, really says more about what type of crowd runs a given race, than it does about me.