Because of the skill levels of the more skilled developer, in a lot of organizations, such a person would be used for more difficult technical work and yet not one that could end up to be boring or repetitive (sounds very obvious, but it can happen that the work that is assigned is not interesting enough and i have known such a skilled developer to want to join other teams when they start feeling that the work they are doing is not really interesting).
One area in which such a developer is used is for new work, or for some new feature where there are expected to be technical challenges and is also critical for the project / schedule. In such a case, one would want the best resources on the team to be on these investigations, since the actual work on these features depend on how well the initial prototype / technical research is done. Once the initial work is done, it might be feasible to hand over the actual detailed work to other members of the team and utilize the more skilled developer for other similar projects.
However, if you want to plan for amount of risk mitigation for the project, one risk that comes up again and again is about serious defects coming up during the cycle, especially near the middle to the end of the schedule. If there is a serious defect coming up in such a phase, it gets more problematic since such a defect needs to be evaluated with a lot of care. While fixing any defect, one needs to be very careful that the fix is done completely and no other problem is caused by the fixing of the defect, and it can and most likely will happen that fixing of a defect will cause some problem (the percentage of such events is low, but not zero, and will happen).
When such serious defects are found near the middle to the end of the project cycle, one really needs to be very careful that these defects are evaluated properly, risks enumerated, and any possible complications reduced or brought down to zero. And as one advances in the schedule, the time period available to do these evaluations reduces. In such a case, it is necessary that the most skilled person in the team either looks at such defects, or reviews the work done by other developers in order to reach the same conclusion.
For this to happen, some amount of time needs to remain free for the skilled developer so that he / she can be put on such defects. Else, if the developer is working full time for some other feature (and it would be an important feature), pulling aside for defect evaluation may not be possible, or may impact the other feature negatively, which has its own impact.