I have another use for tower shield: training your block skill.

Let something (most optimally another player with a dagger) attack you repeatedly while just plain blocking. Semi-to-full AFK. Tower shields work better for this because their straight block value is larger, which means less stamina used on attacks that are fully blocked. Parrying gets double skill per parry, but you get hit less often and it isn't worth being attentive for how long it takes to train.