Battle requires less teamwork than siege.
If you kill someone in battle, he stays dead. You just helped your team, no matter where, how and why.
In siege, killing someone has no value in itself, unless it happens at the right time in the right place.
This is also the reason why most pubbers, including me, don't play siege often - the amount of teamwork needed for the game to be REALLY funny and challenging is more or less impossible to achieve. That's why battle is better suited for pubbers.
Its true that siege takes more teamwork, but the mass coordination required is just not helped by how the mode is set up most of the time... The more teamwork necessary doesnt exactly mean that its better in terms of fun teamplay imo...
I would say battle requires less coordination since its much simpler, but it actually has more tactics-based teamwork in my opinion. In siege, there is less variety; there is a focus on some builds more than others (cav not as used, etc.) The tactics used in siege are pretty cut and dry- group up, either go through front door, ladder up, or backdoor. Seems very linear. Battle can be a camp fest, but it can be considered "tactics."
But overall, battle is the server for direct personal recognition in changing the tide of a round rather than not being able to completely see how much you indirectly help out in siege.