Since enzymes have an optimum temperature at which they are able to catalyze the reaction best and increasing the temperature above optimum can even damage them, how do poikilotherms survive as they change their body temperature according to environment unlike homeotherms. Also wouldn't their metabolism stop with increased temperature?
Instead of viewing poikilotherms as having the same temperature as the environment, view them as lacking an internal thermoregulatory system. Poikilotherms indeed suffer from not having the ability to regulate their internal temperature.
Homeothermy is ecologically advantageous, allowing optimal functioning at a range of temperature. Poikilotherms, therefore, are more restricted in their habitat range, have to depend on external mechanisms for maintaining temperature (eg: Hibernation, Shade-loving lizards, burrowing in desert animals) or simply die due to environmental change (eg: poikilothermic fish are the earliest to die from a thermal runoff into a water body).