There are many niche merfolk support cards that will basically only see the light of day for a product like this, so Wizards of the Coast had no reason to leave any of them in the vault unless they break the blue/green color identity (sorry Sygg)
Lorwyn Eclipsed's Kithkin This Kithkin would crack jokes with you over a pint, and I just love the stuff it's accumulated
That makes playing aggressively against your rival players far less costly, but can also be used tactically to deliberately take characters off the table to reactivate their effects as they enter or leave play - as with Merlin, Goat above
Consuming Aberration is one of the notable exceptions
There are a ton of cards that care about enchantments in both green and white, and cards like Yavimaya Enchantress and Ancestral Mask help to promote building a huge creature that would love to have lifelink and trample
Here is the card as it appeared in Stronghold design: Mind of the Controller WUBRG Summon Legend Gold 7/7 Trample Counts as a Sliver 3: Put a Sliver token into play
