Erza being implanted with the Dax symbote was a emergency situation, Dax would have died prior to arriving on Trill if not implanted in someone, anyone.
Ezra (iirc) wasn't in the symbote program and didn't receive the training.
Oh, ok, makes more sense now. I must have not been paying attention there. ^_^
Well, IMHO,the whole thing of Ezri coming back to DS9 was weak writing from the start. Every other Trill who is joined with a symbiote is supposed to start a new life far away from their former host's life. Technically it was a stretch with Jadzia serving under Sisko in the first place. I think they should have had the courage to just kill off Dax completely.
And as to the emergency transfer to Ezri, I call BS. Riker had one for over a week, and they were able to remove the symbiote without killing him, and he wasn't even a Trill. Ezri surely could have had Dax removed without killing her.
Slightly off topic: Many talks about how Star Trek missed the boat on LGBT issues with Trills and could have done more with them, and IMHO they did. But another controversial topic they missed was that of abortions. What happens when a Trill is joined with a symbiote when they feel they have no choice or change their mind? More specifically, what if Ezri wanted to abort Dax?
Firstly Ezri is a Starfleet Officer - therefore she goes where Starfleet assigns her, not where Trill decides she should go. The only way to avoid that would be for Ezri to resign her commission, something that would go against the whole idea of blending being an equal partnership between symbiote and host. Also, going by your logic, doesn't that mean that if you were a blended Trill, living on Trill, if you had a new host, that host would have to leave Trill?
As to Riker. it's possible that his being a non-Trill made the removal procedure easier, as his body isn't really 100% compatible in the first place.
The Trill issue regarding abortions? I don't get exactly what you mean there. Removing a symbiote isn't an "abortion" unless you're saying they'll just take it out and let it die. It's also of no comparison, as symbiotes are far more than embryos.