Base Defense: Towers Are Not Alwys Enough

Although you can simply say the need for defenders to be able to at least move in times of crisis (excluding the nightelves, who's towers will lift out of the ground to beat them) is a good enough reason for you to have alternate defense, and that those ground support troops have abilities and advantages unavailable to towers, there is one thing that puts the need for them above all.

Bastillas, Steam Tanks, etc, all have greater ranges than towers and can easily destroy them. Always leave a small group of warriors behind to handle that Bastilla. If you do not think your troops could survive beyond that, hide behind the towers and the rest will be taken care of.