Yeah, for storing photos, using the Finder is fine. But if you actually want to manage your photos, then iPhoto (or the like) is required.
I think that people forget (or don't realize) that the Finder is just a graphical representation of files on a hard drive. It's basically a generic database that works with all files. On the other hand, iPhoto is a database optimized for photos.
When I import photos, I do the quick work to tag them as needed with Faces, Places, Keywords, and Ratings. With that work done, I can easily share my photos across my devices: 5-star photos of my daughter, latest vacation photos, holiday photos, etc. Also, iPhoto makes it easy to find my yearly calendar photos (a big deal in my family). A lot of this could be done in the Finder, but not nearly as easily.