in 8.9 Xerath is a complete joke of a champion.
Not sure where to begin...
1.) His only two strengths are poking and lane pressure. The mana changes hurt his ability to do both of these. He might swing his lane a bit harder on occassion, but his mid-game lane pressure is absolutely ruined if you don't buy a tear, and even then - it's questionable at best.
2.) He is easily one of the most counterable champions in the game due to his binary nature. Champions like Nocturne, Shaco, and Rengar can absolutely erase Xerath from the game, and even building well (armor, de-engage tools) there is absolutely nothing he can do.
3.) With Frontloaded damage on the rise, your going to see him be unable to sustain himself in lane and just be a liability to his team because of how gankable he is.
4.) His late game falloff is among the worst of every mage in the game. As he becomes less able to sustain himself both through his passive and late game mana changes, he loses the only thing he brings to a team other than poke, and thats lane pressure.
5.) Out of all of the 'poke' Style champions, he is EASILY in the worst state.