I do not know if the heat spreaders actually help ram tempratures or make it more stable at higher speeds although these are some of the claims, there are even some memory producers who use heat spreaders as standard. I however am not aware of any 'scientific' evidence proving these claims to be true however I am not disputing them.
There has apparently been situations however that once attached if any attempts to move these speaders have apparently damaged the memory chips due to the way in which they are attached
I guess the answer to the question, are they worth it can be answered with another question :- How hot does your memory get ????