Thats another good point. Resistors resist current and shed excess energy off as heat. They are very inefficient and the excess heat should be dealt with in some way via heatsinks or just encasing the resistor. Granted, people dont typically run thier blinkers for long enough for the resistor to get overly hot but in the case of an extended hazard light session, it could pose a problem. Which is yet another reason for the more elegant if difficult solution, get the proper blinker fuse.