So what if Ignite lasted for 10 seconds but did only in total 60 to 400 damage over the 10 seconds?
It would make it a strong choice against sustain champs who sometimes literally only have to press R to start healing everything but remove the damage as being as big of a factor as to why you would take it.
This would make it 40 + 20 per level true damage in total.
Ideally summoner spells should be taken based on the situation. Exhaust can be a good option for a high damage enemy while Ignite would be good for sustain threats and TP would be good for better map control.