Of course Boo is useful, he "Goes for the eyes!"
He is also Minsc second in command, a job which often requires him to squeak. Minsc claims to understand what the small rodent is "saying", although many experts doubt this is actually possible.
I for one think Boo is an essential part of Minsc character... kinda fills in a gap after Minsc's little "accident"
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"Watch your back"