I was mainly focusing on things that people don't use. Exhaust is nearly mandatory on supports or mid laners this patch because assassins.
Ghost isn't as useful as flash. It's largely telegraphed, CC is still applied, and there is a windup to the MS. Flash, on the other hand is unpredictable (they could flash any direction, not just where they're facing), can avoid CC, and is instant.
Teleport takes 4.5 seconds for you to actually teleport. That's enough time for the average laner to walk from the enemy T1 tower to the enemy T2 tower. It's fairly useless in respects of engaging is laughable, as the enemy has plenty of time to prepare for your arrival or get the kill they were after.
I said:
Strong enough to be life saving, provide utility and/or game changing actions.
Keyword is or
As long as it has atleast one of those, it works well. If the only one it has is situational, then it's useless and is never picked. Clarity is never picked because of its situationalness. Cleanse isn't picked either because, not only are there other options, but it's situational. If you were to make ignite do up to 750 True damage, that wouldn't just be a take this. That'd be a "This is mandatory" kind of like release thunderlords. 2 ignites mid game can kill an ADC (assuming it'd do 500 damage (150 base + 33.33 per level).
The point is, many of these summoner spells don't have any of these qualities. Many can't save a team mate. Many can't help engage or disengage. Many aren't as useful as the select few.