As some of you might be aware, I have some very whacked views on Trill symbiosis, how it began, and why it continues. My theories–
Symbiosis must confer an evolutionary advantage (i.e., to further the gene line) to both host and symbiont, otherwise symbiosis doesn’t further the life cycle of either. In other words, symbiosis must make it more likely for host and symbiont to reproduce. Yet, we know that unjoined Trills reproduce, and it appears that unjoined symbionts reproduce. What does this mean?
I suspect that symbionts are pre-sentient before Joining; it’s the exposure to humanoid neural activity that “wakes” the symbiont. (Might Joining confer something to a fetus carried by a Joined Trill female? Perhaps exposure and close proximity to a symbiont’s mental energies affects development of the fetus’ brain. Perhaps the children of Joined females have a propensity toward giftedness in childhood, followed by a descent into madness after puberty. See what I mean about “whacked” theories?)
I suspect that symbionts are asexual. For the most part. There may be some swapping of genetic material between symbionts, but I think that the symbiont simply reaches a point in its life cycle where it’s no longer capable of continuing in a joined capacity, it’s removed from its last host, produces offspring in the chemical pools, and then spends its final few decades in relative comfort in its native environment before death.
I’ve long theorized that the problem with Ezri seen in the recent novels is that the Dax symbiont’s reproductive cycle has been triggered, with debilitating effects on Ezri’s mental state. Given that the symbiont reproduces in the pools, it would probably be very rare for a symbiont’s reproductive urge to be triggered while Joined. Ezri might be wholly unique in that regard.
As for what the host receives out of Joining, I’m still not entirely sure. On a metaphysical level the Joined are guaranteed of something approaching an afterlife. But metaphysics doesn’t further a species’ reproductive strategies.