Ignite does more damage than most abilities
let's compare some summoner spells. excluding flash and smite, as they are must-haves Cleanse: Good if you are against veigar. Useless anywhere else. Teleport: 6 minutes to get back to lane faster. Very useless. Barrier: Stronger heal, but the shield decays after a few seconds. Heal: Small healing and an MS buff Exhaust: Slow MS, the attack speed really doesn't matter Ghost: ms buff that is worse than flash. good on disco nunu, better on hecarim Ignite: Does more damage than most abilities, at level 3, blocks heal, is point and click, and kills over 5 seconds.
Why take anything else when you can take ignite and kill anyone at level 3? It procs electrocute or phase rush, blocks healing, does way too much damage at early levels, and kills you in the most unsatisfying way I love ignite, so much, but if it needs to do so much damage after such a long time, either remove the wounds, or make it scale better. No one likes dying at level 3.