'The whole point of joining is to allow the symbiont to accumulate experiences over the span of many life times. But in order to move on from host to host, the symbiont has to let go of the past...let go of parents, children, siblings, even spouses.'
I thought about that conversation when Ezri Dax pretty much picked up Jadzia's life exactly where she left off, and no one batted an eye about it.
I believe the reassociation taboo is a practical consideration for Trill society, and not meant to be strictly applied to interactions with aliens.
Look at it this way - if joined Trills could just pick up their relationships with other Trills after a "re-hosting", then Trill society would develop into a clique of joined Trill hob-nobbing with each other for centuries. Unjoined Trill would be excluded from such a social circle; just think, how could an unjoined Trill related to joined Trill who are referencing events and jokes from two hundred years prior?
Outside of Trill society, though, the reassociation taboo doesn't serve a purpose, since most joined Trill aren't expected to interact with the same aliens over multiple lifetimes. Normally even a long-lived Vulcan might only manage to be friends with a Trill symbiont over two Trill lifetimes. The fact that Sisko knew three
of the Dax hosts over a couple of decades is a rare fluke. He knew Curzon towards the end of the Curzon Dax era, and met Jadzia near the beginning of what was expected to be a multi-decade joining of Jadzia and Dax. The fact that Jadzia died and Ezri stepped in and did basically pick up Jadzia's social circle is probably exceptional in Trill society and, since they're all non-Trill, not really worrisome to Trill society either.