Many USB chargers provide additional power to charge batteries faster. Less waiting is a clear draw, but many say doing so reduces battery life. Technically it does, but not enough to really matter.
Green energy site Green Batteries explains:
For practical purposes with batteries that are designed to be quick charged, for example, Sanyo, GP, POWERhaus, that is not true. It is important to use a battery charger that has been specifically designed to rapid charge NiMH cells. Actually there is a much greater likely hood of reducing the life of a NiMH battery by using an "overnight" charger than by using a smart fast charger. Overnight chargers rely on the fact that you will unplug them after a number of hours. If you forget to unplug them they can continue to charge the batteries longer than they should. Overcharging WILL reduce the life of batteries. From a strictly technical sense, a battery that is always slow charged will likely last a little longer than one that is always rapid charged. However, the difference is so small that it is not likely to be noticeable for most users.
Basically, stop charging when your device has reached capacity if you want to preserve the battery's life (as we've noted before). If you can't monitor this manually (as most of us can't when we're sleeping), a power-conserving charger can help (such as the Belkin Conserve series). Either way, don't worry so much about fast charging — just unplug your device when it's done charging.
Battery Myths vs. Battery Facts [Green Batteries]