I do think that, to a certain extent, a well-built phase has a core 'home team' audience that it appeals to--some community or Discord, the 'clique' as some call it. I do like Valo's way of describing the situation: you are a guest in someone else's house. Completely come-as-you-go, free-for-all public phase RP has very little you could possibly do if it's not a campaign. You've brought a character to someone else's phase, and it is essentially an opportunity for both parties to get to know each other and see if they want to permanently join.
I don't find it strange at all to host 'private RP' in what is otherwise a public phase, for when quality control is important. I feel like a true private phase is a phase that was entirely built for a predetermined group of friends, with maybe a friend-of-a-friend sometimes joining. Whereas public phase RP would be the idea that anyone can come in and check it out and maybe join the phase. But having a truly 'public' phase is next to impossible to manage without dedicated DMs managing the situation, and really that's not very fun.
It seems like an OK situation. So long as phases accept public people--either by just joining and checking it out on the surface, or through advertisements--the situation seems pretty acceptable. What's important, I feel, is that a person doesn't come online and ask 'where do I go for RP?' and can't get an answer. At least surface-level RP where they can probe whether they wish to make a certain phase their home, rather than just sit in start all day talking with people until someone thinks they're cool and invites them to their clique.