Zhonyas is about 15 times (done the maths here!) better than Abyssal on Xerath. Akali jumps you? Just hit Zhonyas and shes forced to switch target, if your team has an Oracles she'll die. You are probably doing better for your team by having more AP and blowing people up before they get near you. Xerath is meant to burst people down with his ultimate from ridiculous range.