Some places like volunteers, others won't use them.
The problem is that often volunteers will only do the job they want to do, not what neccesarily needs doing. Often they will not annotate or record exactly what they are doing, which can lead to "oops I can't put it back together" moments, especially on obscure a/c.
At Duxford, every volunteer MUST be supervised by one member of staff.
Enthusiasm is great, but sometimes it is cheaper to get the professionals in.