The reason i build this way is because singune pool is bound to get a time cooldown nerf. With the deathcap it is very hard to get killed do to the ability to not get targeted for 2 seconds. This guarentees assists if your team is good and allows you to get mejas charges. The Rylais adds slow to your nukes as well as hp and ap. The ap gets transferred to hp so i say build as much ap as possible. The reason i say get rod of the ages is for the ap and hp it gives, i understand it is sort of a waste of the mana part if you're not for that idea get a lichbane instead. But my recommendation is if you dont want to waste any part of any item, Buy another Rylai's. They aren't unique so the slow stacks and as i stated more hp and ap.

PROS:
Life and AP
Nukes do damage
Useful ultimate in team fights
Takes time but is easy to control a battle

CONS:
No regen, but thats what Q is for.
Can be disabled to death by a good team

GLHF