Gotcha. How about if spammy looking transactions were filtered by each node when they are first broadcast? I suppose ultimately we'd be working toward prioritizing transactions based on their merits... I also see that it's risky to start judging transactions and dropping them, but perhaps there should be an obligation for the transaction creator to be legitimate and respectful of the limited blockspace?
That would have zero effect. The transactions are INCLUDED IN THE BLOCK so if anything, ignoring the transactions in the first place means the node has to request them from the other node that sent it the block.
Now if you're talking about local block generation mining, bitcoind already does extensive ordering of transactions putting spammy transactions ultra low priority and likely not even stored in the mempool, so there's little room to move there. Filtering can always be improved but the problem isn't local block generation but block propagation of a intensely slow to validate block.