Yes. But given a botnet can overwhelm your server anyway, Puzzles let you stay up for others with first class computers. It's not the right tradeoff every time

Yes. But given a botnet can overwhelm your server anyway, Puzzles let you stay up for others with first class computers. It's not the right tradeoff every time, but it's a good tool to have for many deployments. Oh, and as to revenue: if this is useful, ads can get your browser to do profitable work. That's not a risk of want to take by designing, say, Bitcoin mining into this.