The Power of Preview
Adam C. Engst and Josh Centers:
The first option for how files should open is often confusing. Let’s say you have a Take Control PDF book open in Preview, and you double-click a JPEG-based photo in the Finder. It will open in the same window as the book!
And if you select the third option, when you select multiple images and open them in Preview, they all open in separate windows, which is overwhelming and makes navigation far more difficult.
The default option, Open Groups of Files in the Same Window, is generally the best choice.
I agree, but I often wish I could override that to open a specific group of files in separate windows. In particular, I usually want PDFs to be opened in separate windows, especially if they are multi-page. Navigation and removing pages is easier this way. I suppose this would be easy to do with a script, but it would be nice if it were built in. I almost always want groups of images to open in the same window, though.
Update (2016-03-14): I wrote the script for Preview.
"The default option, Open Groups of Files in the Same Window, is generally the best choice."
To me, that's a symptom of how outdated the Mac's window management system has become.