The trade-off is performance vs. both battery life and heat. As the frequency increases the processor produces more heat, and the processor uses more power the faster it runs.
In a perfect world, heat itself wouldn't be a problem as your systems fans would speed up to dissipate more heat--but this would consume even more power from your battery. Some systems aren't effective in conducting heat away from the systems circuits. In that case lowering the frequency can help keep the laptop cooler.
Poorly controlled heat is a problem in two other ways, it can make the laptop uncomfortable to hold, but it can also reduce the lifetime of the laptops integrated circuits. Some systems will shutdown when they get really hot, but that doesn't mean the integrated circuits lifetimes haven't been shortened already. The operating range of the integrated circuits in a laptop vary and are set by the manufacturer.
ACPI (Advance Configuration and Power Interface) is the technology controlling heat and power in modern computers.
acpi command from the acpi package can help control and monitor power and temperature in a laptop.
acpi -t for example will give you its current temperature in Celsius.
The indicator-cpufreq package provides an indicator for the top bar on your system that will tell you how fast your processor is running and help control it depending upon whether you want performance or battery life at any particular time.
Obviously overclocking your computer will require even more power for both the processor integrated circuit and the cooling fans.