Indeed - clearly not a weapon, and not of any value in a more conventional fight. Otherwise, Locutus would not have been confused by it!
Locutus wasn't confused.
Picard had been briefed on Shelby's plan to use the saucer as a distraction. So when they did separate, the Borg used Picard's knowledge and said, "Oh, the saucer is just a distraction, and we can safely ignore it, because the stardrive is a bigger threat." Riker was counting on this, which is why Data and Worf's shuttle was launched from the saucer - the Borg were ignoring it, "just as you should, captain." In this sense, it was actually the stardrive that was the distraction, making the Borg think that the Enterprise was trying to damage them or destroy them when the real goal was to kidnap Picard so the Enterprise crew could try to get Picard's knowledge of the Borg in an effort to find a weakness they could exploit.