Argies are extremely useful. Although it's slower than a pteranodon, it has much more stamina and carry weight, making it a useful cargo bird. You can also go on metal runs, where you can carry an ankylo with an argy and fly towards metal nodes.
Getting a massive TLC update in March 2018. The saddle will act as a mobile smithy, Argentavis will be able to carry with Talons as well as with its beak, reduced carry weight on obsidian and a few other unannounced resources (similar to the mammoth and wood), and will also gain a Rapid HP Regen upon eating (likely from devouring corpses and not specifically food from inventory). Like so people can see!