The mempool doesn't only have one purpose. It also exists to keep unconfirmed transactions in ram so that they can be more quickly relayed to your peers. If you want to free up ram that would otherwise be spam-occupied, and prevent consumption of bandwidth that would otherwise be consumed relaying ram, then your mempool filters have another purpose besides preventing transactions from getting into the chain: they solve the technical problem of spam being in the mempool itself, where it would otherwise consume your ram, eat part of your bandwidth, and pointlessly add wear to your system.