Right now it is a strong signal that you are legit (almost all spam is _not_ NIP-05 verified). If it is from a legit domain, even more so. NIP-05 domain operators have some motivation to not verify spam accounts. I don't expect to see spam being a problem from nostrpurple.com, for example.
Discussion
But there are good people without NIP-05 verification. Are they being filtered out?
damn it! I said verification. I meant "without a NIP-05 mapped domain"
For my relay, I have higher rate limits for nip-05 pubkeys. It is just one signal among many and reduces false-positive spam hits; I would very much like to keep things open for any non-abusive account, regardless of nip-05. I do monitor the rejected events, and am fairly confident I only kick out spam (as defined by me).