The manual states that using a shield with unarmed combat will hamper
the effectiveness if your unarmed combat. If that's true, why do
troll-fighters with claws start off wearing a shield?
Because fighters are a highly defensive class; they always start with
a shield and (if possible) armor, even if it hurts offense a little bit.
(One-handed clawing is still quite effective, just not quite so much.)
If you want an unarmed fighter with no shield or armor, try a monk.
Stefan