I voted for modularity. Being able to have a HUD that remains through the game without having to manually code them into place would be a huge time saver for example.
And as for multiplayer, there is already a functioning plugin by zack0wack0 if I remember correctly.
It might not be the most optimized thing in the world, but I did create a multiplayer game with it and node.js , it was playing fine with 4 people (well, 3 and a local player). Granted there was a lot of client side prediction and it would have probably exploded with 16+ people, but it was playable.
Anyway, like someone above me said, you use multiplayer in some game but you would use modularity in all of them.