thing is, you can't, the impact of the rune isn't big enough to outwight the risk of going for an AA in most matchups, and if it is you're opponent won't just stand there and let you AA him, he's probably sitting under his tower anyway waiting for the wave, and in teamfights you will most likely drop your ult and the flamespitter befor you even get it charged up.
The conditions for the rune are pretty inconvinent for most champs, and even if the effect would be strong on them, they can't use it very effectively in a realistic scenario.