RenderContent Initialize - should be virtual or not?

This is a bit confusing:

Help comment for RenderContent.SetChild function:
// Summary:
// Set another content as a child of this content. This content may or may not be
// attached to a document. If this content already has a child with the specified
// child slot name, that child will be deleted. If this content is not attached
// to a document, the child will be added without sending any events. If this content
// is attached to a document, the necessary events will be sent to update the UI.
// Note: Do not call this method to add children in your constructor. If you want
// to add default children, you should override Initialize() and add them there.

So it says adding default Child content should be done via overriding the Initialize() function. But Initialize is not a virtual function. So what is the correct way to add default child contents?