He didn't taunt him because for Jaredan Aurelion Sol sees Xerath as a rather interesting opportunity. He seems like he respects Xerath compare to other ascendeds. What do you think? They also have common things like being energy and subdued.
I think that of all the Ascendants, he can at least empathise with the whole "being a slave" thing, which might colour Aurelion Sol's opinion more positively for Xerath in particular. In terms of power I can see the Ascendants as being comparable to the Aspects, and considering Xerath's ascension brought him closer to the true source of power for that kind of transformation, he might even surpass the Aspects, I mean if we consider that the Aspects are somewhat celestial in their powers and constitution, and Ascension draws upon the power of the Sun, Xerath's arcane might could put him as being closer to that unbelievable cosmic power we see in Aurelion Sol and Bard than the Targonians.
Of course this is speculation; we don't actually know how the Aspects work with celestial power, or how physical they truly are. Is it a Bard situation where they're more or less completely ethereal, or is there some degree of physicality to them, bringing them closer to Soraka? Dunno just yet.
You also forgot to mention that Aurelion Sol has implied he intends to incinerate the planet for Targon's infractions against him, and it may not be in Bard's best interest to release a dragon that may exterminate a planet he is protecting.
