Its ability looks pretty insignificant at first but combined with certain cards, it can help you produce infinite mana, buff your creatures with a gazillion counters and draw at the same time, or even have infinite turns
Kiora, Sovereign of the Deep is more concerned with slamming octopuses and serpents than it is swimming with other merfolk, which makes it a technical merfolk commander , but not a great commander for merfolk
When Aether Herder enters the battlefield, you get two energy counters
1 Shriekmaw
Newer generations have a different set of grails, and their collective demand can make a modern card with a high population more sought-after than a technically rarer vintage card that doesn't have the same emotional resonance for them
As always, take this information with a pinch of salt