Sometimes network code is easier with deterministic engines. Some engines simply replay the remote/network inputs as if they were local (with various strategies for handling the time sync/time delays), and rely on the property of determinism of their underlying simulations to avoid divergence in player world states.

Knowing only how much Army of Two stressed its cooperative network play nature (including in its title), that's my best lay guess of the strong reason the game wanted deterministic simulations.

The impression I got from dev team discussions/features on The Halo Channel/Master Chief Collection was that Halo was built that way: its network engine wanted rock solid deterministic physics, so all the replay/recap features added to later games were a "free" bonus they were able to build on top from that earlier netcode requirement.