The LM317's high intrinsic current-limit value means it may be under severe thermal stress during current limiting, so that the thermal temperature-limit circuit is also activated. There's some evidence the chip's life is shortened if it spends lots of time at the thermal limit temperature. Furthermore, other components in the power supply (transformer, rectifiers, etc.) are also more severely stressed when at the chip's intrinsic current limit. OTOH, if an external current limit circuit is added, this circuit will be under severe stress.