Why do some Managers prefer Individualization than team-based structure? What might be the disadvantages of work-teams?

Some Managers prefer individualization as they trust few rather than trusting the complete team. Those types of efforts make other team members as freelancers and more dependent on others. These types of efforts increase burden on one and importantly make him feel outrageous. These types of initiatives take away a sense of responsibility from other team members and makes one blued eye. In other words company is underutilized it's resources and limiting other individuals at a minimal threshold level which is obviously not good for individuals in their carrier path. In simple words paying for 5 and getting work from 1.

If the question is for specific cases:  For example, in cases where the deliverable is highly confidential market research or defense related work, it may be prudent to individualize and ensure no one person knows the entire system. Another example is the nature of work, if it is a highly technical or research focused short-term deliverable, then probably an expert in that area is good enough to complete the assignment.

If the question is geneirc in nature, there are many straight forward cases where in volume of work (in terms of effort) is small - where it is good to assign the work to an expert and assign verification/testing of the deliverable to a verification/ test expert. Secondly, if the work involves diverse fields involving multiple disciplines, one person may not understand the other's area e.g., an electronic engineer may not understand the work of an industrial engineer or mechanical engineer - work is divided with the help of an expert architect and distributed to individuals and integrated at the end with expert's help. Thirdly, if the production is well understood with a strong system in place with experts for each production process, and interfaces and integration are well automated with strong documentation standards, then individualization will work very well.

Disadvantages of work-teams: Additional effort to be spent to ensure no mis-communication, ego clashes among team members, strong differencs in opinions, need for a solid team leader to manage the team, ...

Advantages of work-teams: Different perspectives into solutioning and process, availability of a team player who will pick up the ball and run when some one drops, Knowledge/know-how back up across team members to ensure capability remains with the organization, high quality deliverable in time with team synergy, with different team capabilities and roles, solutions to complex problems may be identified in a short time,... and the list goes on ....

For specific task where single highest concentration, punctuality and loyality is a must, only in those areas, team work is disadvantages.

Individualization preference to  do work scenario comes in place when the quantum of work request received from the customer is low with short turn around time. Because of the reason stated in previous sentence the manager may pick an individual who is master in particular work, by doing so you can assure that the work is getting completed with minimal chances for dis-consistency of guidelines/techgnical issues throughout the job.

By doing the same job in team, there are chances that different people have different ideas and the final work may not be consistent and you may need time to review and correct the work before delivering to customer. Also the manager should take steps to develop more persons to do the job over the period, so that the dependency on a single person can be avoided when the person is not available.

It depends on the circumstances your organization carries some may think individual plays a vital role and some thought teamwork is a much better approach.

I didn't see any disadvantage in a teamwork but I think it is a more powerful way of performing a job, brainstorming sessions are more powerful than an individual approach. Teamwork with like-minded people is a better approach anyway. 

Some managers believes that some employees are stronger than others in specific part of the job. and they distribute the missions individually and later they collect all results together. in their opinion, team work may waste the time.

I don't believe this is correct, because when we involve employees to do it as team, knowledge will be shared and even the weakest member in the team will be enhanced.

disadvantage will appear if we choose team member randomly without any criteria.

team work is a culture, for sure there will be difficulties in the beginning of applying it but later, the results will be amazing because there will be a competition and arguments in the team.

Managers have different strategies to lead and great managers knows the needs of his or her team more than anybody else because he is in direct contact with them. There are reasons why some Managers prefers Individualization rather than team-based structure, but I am sure that this strategy is on a short term basis only. I did this technique when I was handling my Sales people in order to check out their individual strengths and weaknesses. How each of them will react and respond on certain situations that are critical to the team's progress itself and the team goals. Once I have find out what I want I know how to position them accordingly, how to send them into trainings and develop their weaknesses turning it on to positivity. This eradicates the so called "wekaest link"... After that, I was able to apply a healthy competition among them with a technique of rewards and punishment. Team grows more stronger after such startegies.