Except of course that it doesn't "totally eliminate the host's personality", JD5000. If that were true, then Jadzia would have ceased to be Jadzia Dax- it would simply be "Dax".
I would also venture forward that, the "hard drinkin', womanizing" lifestyle Curzon lead was indeed tempered down by the addition of Jadzia- in fact, we see, especially in the early seasons when the two were getting "acquainted", so to speak, that the Jadzia part of the symbiont was clearly in many ways in control. I saw no "hard drinking" or "womanizing" (mannizing?) from Dax during most of DS9's run.
If you're making the assertion that the symbiont should have stepped in and stopped Curzon while he was engaging in his lifestyle, that goes against your original premise that the host's personality is "totally eliminated", does it not? On the contrary, the host personality becomes dominant in every instance, while at the same time the merging gives them the memories and experience of the symbiont.
In fact, your own other example -where the symbiont is unable to control Joran's behavior- just goes to show just how much the host really is
in control, despite going against Dax's wishes. I certainly didn't notice anything "gleeful" about Dax reaction- on the contrary, the symbiont clearly is disgusted and ashamed by the behavior of its former host.
As for "The only concern Dax has for humanoid life is when it means it's own survival"- I can't agree with this at all.