Droppers only drop items. Dispensers can use usable, supported items. Droppers won't be given special actions for items, because it is meant to replace Dispensers for dropping items while expanding on the Dispenser's extra functions. I hope they make Dispensers unable to drop buckets - make that a Dropper task. That way, picking up source blocks will work better. It won't accidentally shoot empty buckets into lava or something.