I agree with the fact that his current ult is good, and i like it, but to me it just doesn't feel like it fits with his kit...(at least in my playstyle, I play him as a poke champ not a burst champ).
So when I set up for a siege I want to be able to pump out as much damage as possible without having to wait/relocate for my one skill to come back up, because if i use my ult, then it won't be up for the inevitable team fight that follows.
As for masterdragon481 idea of xerath depending on his ult burst damage to poke someone down, I really dont see xerath as a burst champ, he is supposed to sit outside their range and just keep whittling them down, and if theyre dumb enough to stay, they die, because of the spammability of his damage, sure its not quite as high, but do you guys remember ap kogmaw at all? When I saw froggen playing it, it was scary as ****, granted not everyone is as good as froggen, but still i think the spam would benefit the playstyle of an artillery siege mage more than just three shots every so often....
i dont know just me throwing out ideas to try and get one of my favorite mages back up to snuff... thanks for the responses guys