Its berry collecting abilities are incredible, and that includes seeds as well. Thanks to my new stego, I have seeds for all the vegetables and hundreds of narco and mejo berries for my use (and all other types too). He was encumbered by all the berries he gathered. I think that says a lot about stegos.
I think its best use is harvesting berries. It is basically the midgame trike, although i like the trike more with combat. The stego has a slow attack, but is has a large hitrange, and so it harvests lots of berries at once. It also has a decent carrying capacity. I suggest to upgrade weight and melee.
Once you manage to sit through the tame, these things can be absolutely amazing for gathering berries.
You'll need two people, a quetzal, and your shiny new stego.
Have one person on the quetz, one on the stego, and have the quetzal pick up the stego in its big chicken feet.
The quetz may now fly around, very close to the ground, and the stego can swing to get constant berries.
Don't spam the "swing" button, or you won't gather anything. Wait until the swing animation is over, and then tap it again.
When the quetzal is encumbered, they can drop the stego to regain stamina, the person on the stego can drop the undesired goodies, and you can go off again.