In my personal experience, job descriptions have been something that is both very useful and, at most places, could have been defined a lot more clearly. In the cases where job descriptions are not clear it seems that there are a couple things that happen. One thing that seems to happen when job descriptions are not clearly defined is that people do not do the full job they are supposed to. The other thing that happens is that someone else has to do a job that someone is not doing.
One of the things that is difficult about maintaining job descriptions is that people do not always enjoy the full scope of the work they should be doing, so they do the things they enjoy and not the things they don’t enjoy. When working as a team, it can be easy for employees to get away with this as their teammates will cover for them and be forced to pick up their slack. When working individually, this problem stands out more as there is nobody to cover when things are not done.
Another problem with not having clear job descriptions can be that employees are doing more than they signed up for which is not bad for the employer, but in this case, they may not be getting compensated properly for the work they are doing. This often comes when it is a good employee, and managers are trying to offload work onto someone reliable.
I think that a good way to keep clear job descriptions is to make sure that the company has sufficient staff for the workload because when people start getting too busy, they may pass work off to others or not be doing some of the parts of their job that are not the biggest but are still important to do.