" I assume Dax thinks 'Frig it, I've had eight lifetimes, I'll have another at someone else's expense"
To what do you refer? I'm not really understanding this sentence, although I would like to. The symbiont does not eliminate the host personality- it's more like a merging of thought and experience. Each lifetime becomes added to the symbiont's experiences (unless it is actively suppressed, like Joran was). The host does not vanish, he or she becomes a pair in mind and action.