ADC with Ignite, Support with Heal?

probably best because sometimes the support accidentally KS with the last tick of ignite

and it makes sense being they are the main damage dealers anyway

Because Ignite is overtuned

Because there was never any actual real "reason" as to why the ADC had to take Heal anyways? Especially with all the bonuses to healing that support items give.

I do it because my friend that i Q with most ALWAYS takes ignite, and it's fine by me

Makes more sense than ADC Heal and Support Ignite. Who's gonna be in range to ignite most often? The ADC unless it's a hard engage tank support.