At least in their current implementation they are broken. It doesn’t matter if you like grinding or if you like questing - if they stay this way the game is going to become everyone looking for that golden ticket and logging their locations and spawntimes. The game will entirely revolve around finding them, its just too inefficient not to.
Imagine one person grinding for an hour in a spot to get 25% of their level. Now another person in the same area sits afk or runs errands in town for an hour, but they know that a blue mob will spawn there after that hour. After the hour is up they kill it and get the same or more xp. Once people learn the ins and outs of the blue mobs everyone will just go do something else productive until they know their blue mob timer is up and then go pounce on them, or even bounce from one zone to another to hunt them all down.
Good example in post below mine.