So, why should you undervolt your CPU? What is to be gained and what is to be lost?
If you perform the undervolting correctly, calmly and if you decrease the CPU voltage step by step I can only say this; you have nothing to lose! As an example: my fine computer, an Acer Aspire 5021WLMi has an AMD Turion ML-28 2800+ (1600 MHz, 35W). The default voltage of that CPU is 1.45V. If I undervolt the CPU and lower the voltage to 1.30V. This will lower the amount of heat the CPU emits. This will result in a cooler system. And heat is always a problem when it comes to laptops.
So you get a colder laptop, which is nice if you like to keep it in your lap. The second problem with laptops is the battery time. When you lower the voltage of the CPU, it will not consume as much current! This of course leads to a longer battery time. In the end the difference could be a whole lot and were talking ten (or so) minutes and not a few trifling seconds. Would you say no if you got more time in front of your laptop?
Looking at undervolting in a long-term, a cooler CPU should give the computer a longer lifetime. Of course, we can’t know this in advance but there is always a possibility that the computer lives on a few years longer.