Yes, I see that this would be valuable. It's something we may add in a future update. Originally it was avoided because it looked and felt too similar to the way other apps were predominantly organised through a folder/icon style of navigation whereas we wanted to use fuller screen views whenever possible.
Similar to my response to the thumbnail buttons question. I have a concept design for this.
A few people have said that so we'll see if Apple will let us do it. The last year I've had to do a fair bit of contract work to pay the bills so a lot of work has been started and not finished. Thanks for your thoughts.
I spend a lot of time with clients who are after images for office/home and so use large galleries of images for them to browse looking for something that piques their interest. The current pull down thumbnail view is useful to an extent, but what would really help navigate large galleries would be a full-screen thumbnail view that can be scrolled through, much like the standard app and other gallery apps.
For example someone could be looking for a image that has a particular dominant colour to match a wall colour. Whist the current thumbnail view is functional it doesn't work well with galleries containing 100+ images. At the moment it is often easier to come out of the slick, professional, Foliobook and just use the standard app.