C2:Spread the Blight (saves-1 Hamp-20 debuff, stacks, unknown limit) might be the reason, especially if it stacks high and quickly from a no-save spell; eg. from PL:Wiz MM. If the build is run by established characters, I could see a L3 splash for a one-click saves-10 Hamp-200 and that could be doubled up with debuff swaps.
It's not the L2 spells (nothing special there, especially without spell-levels) and I doubt it's T3 -- nothing there for non-BC -- unless there's a bug; eg. if out-break works on more than it's listed spells, T3 BC would become very valuable.