You snipe at 5 minutes. Now someone else snipes at 4:57. They only give up 1% of rewards but gain far more than that by being ahead of you. You now decide it is worth it to snipe at 4:54. Repeat. As a result, much of the curation rewards on useless curation is then returned to the pool to be used elsewhere.

This already happens BTW. Many votes on known-payout posts are made around 5-8 minutes I believe (I haven't checked the exact numbers recently) even though the penalty timer currently runs to 15m, returning curation to the pool. That part won't change much with HF21, although the higher absolute curation rewards may drive a bit more activity.