The goal of downvoting is not to "protect the reward pool". it is to direct the reward pool elsewhere, when the downvoter doesn't agree that payout is a good use of reward pool funds. The reward pool payouts are the same regardless, it is always paid out, the important question is where.
RE: The main excuse for malicious down-voting is "Over-Rewarded Content..." What about the account making $10k+ PER DAY in spam comments?