It's useful for players who speed breed, have 3 to 4 nests, and need to collect every dragon available before the dragon expires, and of course have lots of expendable gold
The second den would allow the caretaker to continue breeding without speeding up the long failure on the first den (33hr virtues or 44hr diamond/crusaders).
I honestly don't see much value in it. But for a high level collector it would increase their breeding chances and save some gold by NOT speeding through long failures (33hr virtues and 44hr diamonds come to mind).