It is not always obvious how to work together as a team. People experienced with solo projects often falter, or slide into inaction rather than step on their co-worker's toes. Larger teams give some members an opportunity to hide - or play defensive "not my problem" games with their peers. All of these things hurt your efficiency and overall productivity.
But the biggest team-killer is having a member without clear responsibilities.
A lazy or ineffective member can be replaced. An actively malicious member can be identified and removed. But an eager well-meaning team member without a specific goal will careen from subject to subject, adjusting and influencing your project unpredictably, and the results are predictably disastrous.
Why should this be the case? I believe it happens for a very simple reason: because people without clear goals create their own high-level goals, which will tend to clash with your current direction. I always advocate for including a vision-holder on your team, and your vision-holder should move between groups spreading influence. But if un-tasked team members act like vision-holders, you end up with conflicting influences.
This might sound like an issue that only happens on large teams, but I've had this problem in teams as small as four. Maybe an artist doesn't have any deliverables while you are doing initial tech-exploration, or a designer isn't provided with enough direction to stay in sync with management.
At this point some of my more cynical readers might suspect that I'm poking fun at producers - who often suffer complaints that they are superfluous to the project. But there is a big difference between "produces no tangible assets" and "has no clear responsibilities." Every discipline is, sometimes, responsible for tasks which produce no clear results. Programmers might take weeks to evaluate dozens of tools - which feels like wasted time to many. Designers might document features which are immediately cut. It is the lack of goals, not the lack of results, which kills teams.
Why would a team put up with a superfluous member? There are two cases I've had direct experience with:
1 - The person is very friendly, and pleasant to have around.
2 - The person is blackmailing the studio.
Number 2 sounds like an extreme case, but it's actually quite common for an experienced team member to demand assignment to a particular team, or demand to be left off a particular team. It's often easy to simply let them have their way - they are experienced and valued members of the studio, after all! But if this placed them in a position where they have no clear goals, the weight of their experience will drag down your project that much faster.
Studios tend to use a self-reporting method of task-tracking, which is a poor way to identify which members of your team might be drifting without goals. Management should always be able to draw a direct line between the goals of the project and each member's current task. Period.