There is no reason to not take ignite on a support now, i don't know why they decided ignite needed more damage and heal needed a longer cooldown, but now ignite dominates bot lane and it isnt a choice of which one you want, its take ignite or lose lane.
Ignite gives kill pressure, exhaust takes it away. The ability to slow and reduce an opponents damage might not seem as good at first but if you use it ar the right time it is far more effective than ignite or even heal
Do you NEED ignite to secure a kill? Yes or no.
Ignite doesn't help a lane with zero kill potential and can't shutdown fed people later on.
is meant to be a defensive CD to reduce incoming damage. That is why they removed the AR/MR shred it gave so it wouldn't be used so offensively except as a slow. Good against burst champions and counter measure of all-ins.
