Yeah I like to start stacking Rabadon's when I find myself a side character in some other fed champ's carrying showIsn't there diminishing return on having so much AP? The money would have probably been better spent on AP items with other effects like Rylais or Zhonyas unless it's a guaranteed victory and you're just doing it for fun.
Of course, I build double Philo Stone and double Shurelyas so what do I know.