Fun fact about Thavarian Onions (not fun)

[Tiggy]

for a bird that's really only going to be as good as a rank 10 chocobo, just with some added flexibility

False. Each tree has a number of passive stat increases that make the choco stronger at all times. Not just when changing stances for flexibility. They do actually gain more HP, Acc, Crit rate, etc for each tree completed.