I haven't noticed them being particularly good at spell selection, but it's probable that the strategic AI scripts mages (it has a high tolerance for micromanagement ), which means that if you leave your mages unscripted it will be "better" for the first five rounds.
-Max
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Bauchelain - "Qwik Ben iz uzin wallhax! HAX!"
Quick Ben - "lol pwned"
["Memories of Ice", by Steven Erikson. Retranslated into l33t.]