Larian Studios Forums Divinity - Original Sin Divinity - Original Sin - General One-handed + Shield vs. Two-Handed.

Generic Str-based character = Two-handed, hands down
Man-at-Arms/spellcaster hybrid but counting on weapon damage instead of spell damage = Still two-handed
Man-at-Arms/spellcaster hybrid but planning on spells for most damage = Shield is actually the right choice

In other words, shields are good on armoured spellcasters, but not on warriors who happen to know a few spells.