I guess it's the good ol' convenience-security tradeoff 

because not everyone can set up their own NIP-05 identifier. and if your identity gets rugged, you still don't lose your followers, just an alias

Semisol

Yeah, but why outsource even that? are we gonna see people getting rugged off their  alias?

I'm a little overdramatic, sure. 

But the whole point of nostr is to not rely on single centralized entities for identity and publishing. If your alias is managed by someone else but you, you forfeit some of that. I know you don't give up your private key, but you do give up some control.

gandlaf21

nostr

NostrPlebs.com Nostr NIP-05 ID registration and hosting service

derekross

This does not outsource the custody of your identity, it only allows you to get a NIP-05 identifier (an alias for your pubkey, which is more memorable and short). Your private key is still only known by you.