In the episode "Facets", Jadzia is able to communicate with all the previous Dax hosts. Verad is not represented among these. Is any reason given for this?
There are a few possible reasons why Verad wasn't present during the zhian'tara. For one, the joining process was never completed between Verad and the Dax symbiont. If it had (which takes 96 hours), Verad would have died after the symbiont was removed.
Secondly, the purpose of the zhian'tara is to provide a means for the current host to speak with previous, deceased hosts and gain a better understanding of who they were individually, as well as who they are now collectively as a single joined life-form with shared memories and a composite personality. This helps the current host understand where certain personality traits come from, what experiences have shaped the being that they are now a part of, and what are their strengths and weaknesses as a joined consciousness. It also provides a sense of closure for the current host and is additionally an opportunity to solve inner conflicts between different host personalities.
In light of this, it really doesn't make sense for Verad to be summoned. He was a "Dax" for all of 1 day, so there isn't a lot of shared memories to reminisce, emotional closure to be had or deep introspection that could be gained from his participation in the zhian'tara. Certainly, some of his memories (and at the very least the memory of that incident) would be retained by the Dax symbiont as well as Jadzia, but the joining was so brief and incomplete that Verad wouldn't have had much influence on Jadzia Dax's personality.
Lastly, Verad is still alive. So if Jadzia really wanted to talk to that host, it would make much more sense to talk to him directly rather than conjuring up a psychic projection of his personality from the 1-day joining.
The same would likely apply to other temporary hosts like Riker. You wouldn't expect Riker to be summoned if Odan's future hosts perform a Zhian'tara. I would compare the effects of Riker and Verad's joinings to a Vulcan mind-meld. There is a temporary joining of consciousness that also transfers lots of memories and inner thoughts to the other individual. But it's not quite the same as a katra transfer, wherein the full consciousness of the source individual is transferred into a new host.