The tradition FF summons are on the sky region of Lemures, Leviathan specifically dwells there. Since the auracite of that game are in fact the Zodiac stones, it makes for an interesting explanation how the stones and ff-summons got to west ivalice. As far as Bahamut, his origin is revealed in the final battle of XII.
I wanna come in here in defence of FFXII. The Esper system is representative of the game itself - in that people came into the game expecting something completely different and as a result, a large amount of people got turned off from the game (including me initially). I can delve into this more, but that's for another thread.
FFXII is all about strategy, and as such, you have to know how to work the Espers to your advantage to make them useful. Unlike previous games, they're not safety nets that you could fall back on to overpower your enemies - you have to put some thought into it, and in doing so you'll be rewarded with powerhouses. My first two playthroughs I found them useless but on my current one I have to say they are extremely useful.
The main problem is keeping them alive, and since early on you most likely wont have more than 1 mist charge, it can be a major turn off when you don't have a way to heal them. Potions are a must. If you can do that, then they will cause crazy damage over and over again climaxed by a huge attack. And if you can use the right ones to exploit an elemental weakness, it makes for an incredibly easy win. Granted, eventually they will start being killed off easily even by regular monsters, but at that point you need to go out and get the 2-mist charge ones, and then eventually the 3.
Like I said, they're not the overpowered trump cards of previous installments - you have to put some thought into it. But if you do you may find it very rewarding. Like others said, the backstory of them was very interesting; it's kinda eerie how the scions helping you in your struggle in XII are the demons who were trying to resurrect their fallen leader in Tactics.