Most of the Apple-branded Mac OS X applications, such as Mail, GarageBand, iMovie, Pages, and Keynote, have a Media Browser that you can use to place photos into your messages, projects, or documents. It can sometimes be a handy way to include an image or audio file without opening another application, exporting the file and importing into your new document. The Media Library will show images from iPhoto and Aperture. I would recommend that Apple add the Photo Stream to this list of photo sources.
This morning, I took a photo with my iPhone and wanted to include one of those photos in a lengthy message I had composed on my Mac. I opened the Photo Browser, pictured above, hoping that Apple had perhaps made the Photo Stream as one of the sources. Sadly, that was not there. I had two options. The first is to open Aperture to synchronize the new Photo Stream additions. The Photo Browser will then show a Photo Stream from my Aperture library and I can add the photo from there. The second is to save the message on my Mac as a draft. On my iPhone, I would go to the Camera Roll and copy the photo. Then I would open Mail and retrieve the draft. I would then paste the image in the message composer and send it. The first is a much easier process than the second, but I think including the Photo Stream in the Media Browser would make this process a lot easier.
To implement this, Apple would have to move the access point of the Photo Stream from the desktop apps, specifically iPhoto and Aperture, to the OS, presumably through the iCloud System Preference pane. This way any application can use the Photo Stream, perhaps even non-Apple applications. Personally, I would really like it if MarsEdit could tap into the Photo Stream so I could add Photo Stream images to a blog post.