Indeed. Let's say your AD is going for a tank, Wither the tank and use spirit fire, it will make your carry's job much easier. I play Nasus all the time, and I consider my E an utility more than a weapon.

your missing the point of spirit fire. the danger of it is not the damage. it's the armor removal. usual "burst" is E,Q and W when poor enemy are trying to escape.

I used your build. Just amazing, Free wins :) thx alot, waiting for more builds of other champs

Nicely done, 1+. Check jhoijhoi's making a guide - guide. It will help you a lot.
Anyway, can you find time to check my guide? U may like it

actually spirit fire synergizes quite well with both wither and ss

This guide is far from finished.
For one thing you have only 5 items at the end of the game, for another you need to elaborate more.

Add some coding and colors, pics, etc.

Not worthy for a +1 quite yet