IT support skills matrix

If you work in a managerial role within an IT department, have you ever had to put together a 'skills matrix' for an IT support team? And if so, how detailed did you go in terms of skills areas of the individuals being assessed? Or did you use broader categories?
Did you think the exercise was worthwhile?
And what specifically came out of the exercise (if anything)?
How often do you refresh the matrix, e.g. is it an annual review, or more frequent, or does something trigger a review etc?  
It would be interesting to learn if there are particular skills areas that are generally the least well covered & transferrable to others e.g. highly specialist skills that are indispensable, and difficult to replace/teach internally, or by recruitment.