Well, depends on how much they increase the heat dissipation area of the memory chips' surface and how good the airflow around them is.
What they definitely do is that they even out temperature changes of the memory chips, making the increase in temperature a little slower - assuming the thermotape attachement is of any good quality. Also, if they sufficiently increase the dissipation area, they do also increase the rate of heat removal. Again assuming the thermotape connection between the chip and the heatspreader is any good.
Of course, on default clocks they aren't necessary at all...