Could Bitcoin dust spam be effectively combatted by actually including it in the block-chain?
why not do include such dust, so that spammers are harmed economically
  and stop doing it?


I'm still not sure that would be enough to prevent this kind of spam activity. Even if these transactions were committed to the blockchain, 100,000,000 such broadcasts could be made from 1 BTC, that's a lot of reach for $700.