What is the difference between using a PropertyDescriptor that returns a value for the IsReadOnly() method, and one that is associated with a ReadOnlyAttribute?
The main difference is that this allows you to seize more control if you provide your own PropertyDescriptor implementation (via ICustomTypeDescriptor, TypeDescriptionProvider or TypeConverter). Then you can choose your own logic for when it is writeable - for example, based on access rights.
But yes; under the default implementation, it will report read-only for properties without setters, and for properties marked with ReadOnlyAttribute.