Using Finder view and toolbar plug-ins, and recalling the presupposed OS-level support for powerful, high-performance, arbitrarily extensible metadata, it's only a small stretch to create a saved Finder Browser window that is, essentially, iPhoto....It's a win-win situation: iTunes and iPhoto get slimmer, and the Finder becomes much more powerful.
This is where Siracusa gets into trouble. Spatial vs. Browser issue aside, since we're talking about non-power users here, I think the big question is how much should these users be using the Finder anyway? It's natural for most experienced/savvy users to gravitate towards column mode, but for new users, the iApps stand a chance of making using the Finder a rare event.
What I mean is that, if you want to browse your photos, use iPhoto. Finding pictures based on their thumbnail is far more intuitive than remembering that the photo you took a few days ago was auto-named by your camera "00020394488.jpg". Similarly with iTunes - if I want to listen to music, I'll launch that and browse through Artist: "Massive Attack", Album: "100th Window", Song: "A Prayer for England"...that's a whole lot easier than going into the Finder and clicking through "Macintosh HD:Usersowellr:Music:Massive Attack:100th Window:A Prayer For England.mp3".
The kicker is that these "Live Queries" already exist in iTunes, and to some extent ("Last Imported Photos") in iPhoto. And since they exist in their appropriate media-browsers, the information/meta-data they can act upon is very specific to the types of files that you need. To create these in the Finder would involve too many criteria for the average user to have to deal with...If I'm creating a "Live Query" folder in the Finder for all the photos that I shot at "f 2.8", I don't want to have to wade through options like "Composer" and "Genre".
These "custom Finders" exist already for photos, music, and email (the three I use most often... I'm sure there are others...), but what's really missing is a nice organizational app for office documents. Perhaps "iWorks" could remedy this (although I doubt it). But clearly there are different criteria for quickly finding office files. In the same way that I remember an email by who sent it to me, I might remember a doc file by the title (inside the document, not the filename) or by the subject (which could be dynamically created by summarizing the document). I might remember a Powerpoint/Keynote presentation I was working on by the look of the first slide (similar to iPhoto).
iApps for different types of documents should include intelligent organizational interfaces specific to those types of documents. The Finder is not going to go away, but it would only be needed occasionally by power-users, novice users would just launch the appropriate iApp and start working.