PublishingWeb.IsPublishingWeb malfunctions on custom site definition

I typically apply a Feature-based approach for the provisioning of a SharePoint site into a full-functioning application . I prefer this above putting everything within a custom site definition. I'm in good company with my resistance against (overly complex) Onet.xml's [Andrew Connel, You don't need to create site definitions, Joel Oleson, Do you Really Need to Create Custom Site Definitions?]. I actually reuse the same site provisioning engine functionality within multiple SharePoint projects and applications. The reuse is made possible by specifying the provision actions via an XML feed file. Via this, the engine can be instructed to provision structure (Lists, Libraries), content (publishing pages, ListItems), and configuration (web.config via SPWebConfigModification, and PublishingWeb + SPWeb properties).

With respect to the latter, I discovered that the OOTB PublishingWeb.IsPublishingWeb sometimes plays tricks on you. Let me explain. In the provision engine I've implemented functionality to set diverse navigation settings: IncludeSubSites, NavigationShowSiblings, InheritGlobalNavigation, and so on. These properties can only be set on publishing webs, and are in the SharePoint Object API available via a PublishingWeb instance. In the engine code I first do a check whether the SPWeb on which the Feature is activated, is actually a PublishingWeb. And only then via PublishingWeb.GetPublishingWeb access a valid wrapper reference, and set the navigation properties.

This all worked fine in situations in which I applied the provision engine afterwards on an already created Publishing site collection. In my current project I do it slightly different. I've implemented a minimal site definition, and within the context of it activate per web in the created site topology the Publishing feature, and next execute the provision engine. In this setup, the navigation settings appeared to be ignored by the provision engine. When I debugged the provision execution, I discovered that PublishingWeb.IsPublishingWeb returned false for all webs in the site topology. Very strange, since I made sure to first activate the Publishing feature...

I ran into the forum-thread IsPublishingWeb() - what does it REALLY check for? posting a question on this issue. And followed up on the advice given there: implement an own SharePointHelper.IsPublishingWeb method to determine whether the SPWeb instance is valid for being wrapped into a PublishingWeb instance.

BTW: My own experiences learn that it is not due the asynchronous site creation behaviour. Also when later via site settings (de)activating the provision engine feature, PublishingWeb.IsPublishingWeb remains returning false, despite that the Publishing feature is activated on the (sub)web. It looks more as if PublishingWeb.IsPublishingWeb is somehow dependent on something that is set behind the curtains via OOTB Publishing site definitions.