League of Legends (LoL) Question: when do i take ignite on support?

Jesus there's so much hatred for supports running Ignite. I don't get it. Like, there's a reason Ignite is used in most supports in LCK. What irks me most is that i feel like this dislike towards Ignite is unconditional.

To start off my answer, i'd like you to re-word your question to something as follows, "When can I really get punished for not having Exhaust?"
It's undeniable that Ignite adds a lot of kill pressure in lane, especially with champions with burst damage, CC, or a combination of both ( Thresh, Leona, Morgana, Blitzcrank are all good examples). The strength of your Ignite is amplified even more so if your carry has high kill potential ( Graves, Corki, Lucian), or your opponent has a carry that can easily be caught (something like Caitlyn or Jinx are easier to catch than Sivir or Corki). That said, I tend not to take Ignite when I am playing a low kill pressure lane, as Ignite basically gets outscaled by Exhaust by level 9 or so.

There are some difficult decisions you have to make though. Like, if you are playing against assassins, you have to ask yourself, "Do I REALLY need Exhaust to save my ADC from Death Mark damage?" If you run Ignite instead of Exhaust in a situation like this, most of your ADCs deaths from assassins will likely be your fault - you may single handedly lose the game for your team.

The reason I see people giving me flak when I use Ignite on supports is that they think the DOT will killsteal, which it has many times before. I run Ignite only when i'm playing an aggressive kill lane vs a heavy healer ( Soraka )

I think it is only useful when you really want to stop some healing from their laners, or to quickly drop a tanky type IE graves. In either case it needs to be dumped early in the fight.

Well, in general you don't want to drop Exhaust at all, because your team will need one to handle any high damage targets that might wreck your team and no one else can be relied on to take it on your team. Even if you don't use it much in-lane, when teamfighting is required, Exhaust will be more necessary, since your team will likely have an Ignite anyway.

Putting that aside, Ignite is to increase kill potential in lane, so you if you do want to take it, it would best be suited to supports who also have a high kill pressure like Annie or Vel'Koz.

In addition, it would be best taken when the enemy laner does not have a strong fighting presence during the laning phase and in all-ins ( Caitlyn, Varus, Ashe).

On opponents like Vayne and Lucian you will need the Exhaust.