An important use of the core is being border router to proprietary software, that is usually indexing the block chain and mempool. That software is also assuming that double spends are not relayed by the core.

To remain useful as border router, the replace-by-fee patched core should only relay double spend if it actually replaces an earlier transaction, as otherwise the replace logic that is according to your commit more than just fee comparison, would have to be replicated in the proprietary stack and mempool might get out of sync with that of the border router.