The callbacks in property metadata are not typically public members on the containing type, so the value of this property is not important for most scenarios that just consume an existing dependency property's metadata. One reason this property is exposed is so that metadata subclasses can perform their desired merge logic if both base metadata and overriding/adding metadata specify a CoerceValueCallback. However, the default merge logic for a CoerceValueCallback is to replace the previous one.
CoerceValueCallback is defined in the object model as read-write. This is so CoerceValueCallback can be adjusted after initialization of the PropertyMetadata object itself. However, once the metadata is consumed as part of a call to Register, AddOwner, or OverrideMetadata, the property system will seal that metadata instance and the properties are now considered immutable. Attempting to set CoerceValueCallback once IsSealed is true on this metadata instance will raise an exception.