If you overclock and use RAM intensive tasks, more heat will be produced, but generally heat spreaders may not be of any benefit to common users and sometimes gamers too because there isn't enough heat generated for it to be useful.
Heat Spreaders were a big thing with DDR2 as they produced large amounts of heat. Not a big deal anymore w/ DDR3. Those big tall, toothy things ya see on RAM are vestigal, still around cause they look cool and increase sales more than anything else. here's an old review from when the "ole days" of DDR3
At more than 2" tall in certain areas the Corsair Vengeance could pose a problem for users like me who use large coolers such as the Scythe Mugen 2. I was able to use the Corsair Vengeance only after I mounted the fan on my cooler on the backside. Size is definitely a concern with heat spreaders of this size and therefore I encourage users to check that they will have enough space under their heatsinks before purchasing the Corsair Vengeance kit.
The problem I have with the Corsair Vengeance is the same I have with many kits of RAM on the market. Companies insist on putting large coolers on their RAM and it limits the choice in CPU heatsinks that can be used within users system. DDR3 does not require these elaborate coolers with its lower voltages which translate to lower temperatures then RAM saw during the DDR, and DDR2 era. Corsair is correcting this with low profile versions of its Vengeance line but ultimately I would like to see the average size of coolers drop instead of having to look for specific low profile versions of a memory line.