If some of the creatures are illegal targets as the first ability of Jade Seedstones tries to resolve, the original distribution of counters still applies and the counters that would have been put on the illegal targets are lost
This keeps Polymorph fodder up, all while having creatures in the air that can attack for damage if need be.
Best case scenario, you remove two small creatures and draw a card
That's very Golgari, but now you trade green for blue, and in doing so you open yourself up to mass card draw, filtering, counters, bounce, stealing, copying, and many more effects
They exile a bunch of stuff from their library
Bringing them Back (and sometimes keep it!): Adarkar Valkyrie Sac an opponents creature, then get it permanently