Microsoft didn't even turn on the servers until the day after we posted our single player review (despite us having reviewed from a boxed copy), and, to be honest, we didn't have the chance to rally 12 TVs and 12 Xboxen (not to mention multiple copies of the game) to engage in a System Link marathon, and we didn't feel that this would adequately represent the 'real world' in any case.
We wouldn't normally treat the single and multiplayer facets of a game as separate entities, but in the case of Halo 2 we didn't really have a whole lot of choice.