What are they like?
Have you got someone in your team who never seems to be part of anything? From time to time they come with an information that might go against what everyone in the team has been saying?
People like this may seem like they're not players and whilst they might not enjoy peer to peer interaction doesn't mean they're not concentrating on the task. Not everyone can be a leader or talker. People who stay back and say very little are often analytical. They are more task than people oriented and that's why they might not seem like team players. However they often come up with valid points even though these might seem negative, because they spend their time thinking about all possible scenarios. They are often perfectionist and will want to present flawless pieces of work. They spend a lot of time researching and making sure what they are working on has as much details as possible. Which might confuse or 'bore' those who work in a much faster pace.
Why you need them?
The answer to this question is obvious. You need to have someone in your team who will look into details of everything you do. Someone who will analyse all the numbers and possibilities so you can avoid failure and take calculated risk because you have all the information to hand. Give them the time they need to work on this details but keep clear deadline agreed by both of you to avoid unnecessary stress through wrongly set expectations.
Have you got someone with these traits in your team? Keep an eye on other personality types working for you or with you...