+1 weapon is a worthwhile upgrade when you're on turn 4. DPS really need to go all out in this fight, there's not much room to hold back for the sake of tanks. On top of this, tanks need to conserve resources, particularly the off-tank. This means threat margins can be very tight. The small boost in threat generation from a +1 curtana makes a very noticeable difference.

I was very reluctant to get my +1 weapon because stat wise other upgrades are nicer, but it changed my threat holding capability on turn 4 from shaky to solid. Though I thought I would, I don't regret it.