I think using the pod doesn't actually gain you an action. If you sprout it at 3 mana I think you just turned your wheels and didn't go anywhere.
It isn't about gaining an action; it is about being able to bank an action for later. In the early game especially, you have to use the precious limited resource of actions without fully understanding your opponent's strategy. With Seedling Pods, you don't need to make an immediate commitment to a particular strategy, and can wait to see what your opponent does without wasting the actions you need to counter it.
At the moment, the options as far as plants go are a bit limited, so it doesn't have the versatility it could. As more different plants with diverse functions appear, Seedling Pods will become more and more useful, since you will be able to respond to a greater array of threats.
In short, if you want to make Samara Tree better, just more plant spells. Lots of plant spells. Plant spells with all sorts of uses; anti-enchantment plant spells (like
Magisbane Lily), anti-swarm plant spells (like
Lychweed), anti-conjuration plant spells (like
Creepervine), anti-nonliving plant spells (like
Sunflower), etc. A full toolbox of plant spells will let the ability of seedling pods to delay decisions really shine, and will make Samara Tree a more attractive option relative to Vine Tree. It doesn't even matter whether the new plants are vines or not; the point is the ability to postpone committing to a particular strategy.