As soon as you try to do anything useful to foo it's not arbitrary anymore. You have to make some kind of an assumption on the underlying type, check for keys, nulls, maybe it's a number (the right number?), maybe it's a list. So now you have to scatter some boilerplate checks everywhere you touch a part of foo.
If you could parse it into a typed structure up front, you'd only have to deal with this in one spot, and have guarantees for everything else that follows.
Bonus: if your typed language has good support for records, you can even do this in a way that only provides structure to the parts you care about, and is robust to changes to any other parts of the json.
As soon as you try to do anything useful to foo it's not arbitrary anymore. You have to make some kind of an assumption on the underlying type, check for keys, nulls, maybe it's a number (the right number?), maybe it's a list. So now you have to scatter some boilerplate checks everywhere you touch a part of foo.
If you could parse it into a typed structure up front, you'd only have to deal with this in one spot, and have guarantees for everything else that follows.
Bonus: if your typed language has good support for records, you can even do this in a way that only provides structure to the parts you care about, and is robust to changes to any other parts of the json.