One reason to favor the dependencies-first approach in Arc is that we have mutability.
If you're only doing single assignment and side effect-free programming, then your code doesn't have a significant order . But insofar as your program is imperative and performing mutations, the order is significant.
A consequence of this is that if you want to be able to take advantage of imperative features, you're making it harder by ordering your code any other way. I say this because even if your code is purely functional right now, when you try to insert some imperative code later, the order is going to start mattering more. And it's going to start seeming tangled and confused if it doesn't build up in the order of execution (at least it does for me).
So dependencies-first programming plays especially well with imperative code. I'm also particularly interested in it at this moment because I'm working on a refined auto-quote mechanism that could be hard to take advantage of if you're not programming this way. ;)