I'll try to make some points here, in a professional perspective.
I'm a national book award (6 books) illustrator and graphic designer. I've done more than 30 books, so I know about the topic. A little.
1. When you talk about an illustrated book, the first thing that comes to mind is for children. BUT, there's also illustrated book for adults, like professional photography books, comics, even portfolios. I do books and I read them. Me, personally, will hate to see thumbnails of a professional portfolio, because those are planned very carefully. On the other hand, you could find great use for thumbnails when you're enjoying, let's say, the graphic evolving of a famous artist: that way you can come back easily, compare styles, how they evolved, changed, progressed... What I'm trying to say here is this: thumbnails should be OPTIONAL.
2. A good designer would try to assimilate this "iconic resource" to their design. In some projects you could find is useful. In others, it's just a **** nuisance. So, again: it should be OPTIONAL.
3. Some of the books I did are educational. Even though we all know we sneaked our books when we were children (and now too), I wouldn't like the student's go ahead of their tasks because they have access to results thanks to the darn thumbnails. So, in this case (that should be OPTIONAL -again-), will help teachers to be very punctual about the thumbnails they want (like the first page of the chapter, or the exercise but not the answer...), but the problem is that this kind of "help" could be a big problem.
4. To convert a paperback into a digital book. ****.. If you already have this gorgeous real book, when you try to adapt it to digital format.... stupid, STUPID thumbnails. They are ugly. They are just there, showing you everything, spoiling the surprise, screwing the original design. Let's make it simple: you already now that graphics helps you out to built some personality. It's not the same a geek boy than a burlesque woman. Try to imagine to build a concept for the geek boy, and you certainly will find thumbnails useful. Try to make an art nouveau burlesque moulin rouge style, and this elements will look awkward. Or to not making it sexy: if you're making a wonderful story for kids about pirates, and maps, and treasures, hiden stories inside other stories, this thumbnails are totally ruin the experience.
5. Some months ago my bigger problem was that I was trying to adapt this thin real (paper) book. Indesing (the logical program for a professional) didn't work for me. So I tried other dumb apps. So then Apple came with IBA. Didn't worked either (because it forced me to work with both vertical and horizontal formats: AND IT SHOULD BE OPTIONAL! as it is now, luckily!). So I tried Pages. Didn't work. So my iTunes tells me that IBA have changed something and that now I can make "Portrait books" only, and I threw a party. Well: thumbnails are ruin my book, the script and the personality of the story. TOTALLY. So IBA still doesn't work.
If I dare to assume good reasons for the thumbnails to be there forever and ever, is that Apple actually say they did this App for teachers, basically. Teachers and professors could design their classes, send this short interactive books to their students, and actually, that makes sense. If I dare to assume more, Apple is not paying any real attention to other publics. Maybe they don't want "wannabe writers" to have professional options (though they say opposite), I don't know.
So basically my message here for Apple is: even a professional find this thumbnails awkward and useless and ugly. I don't want them in my book, they totally ruin the design and the personality. As a professional I can say to all of you that there is NOT A SINGLE APP in the marked (free or costing you $500) that doesn't make it ridiculously hard to make just a simple illustrated book. If is not about the orientation (vertical or horizontal) is the font. Or the exporting files. Or the cover. Or the background. Or the page numbers you can't delete. Or the thumbnails. Or to allow the reader to change the fonts. Or the program to resize a perfectly designed book.
As you can see, there are many, MANY issues they have to solve yet. I understand that thumbnails are a useful resource, but ONLY if you want them (it's like been full of food and yet be force to eat desert: you'll not enjoy it. Actually, you'll hate it). So in the meantime, I'll have to pay a programmer to make an app for a free "book" that doesn't even have any sounds or interaction whatsoever, just because Apple is one of those who doesn't understand that sometimes a simple book is better than an interactive book. Some projects ASKS YOU THAT. Conclusion: totally ridiculous!
Please excuse my English, and I hope you understand my point here. Basically: thumbnails should be optional!
p.s.: thanks (Apple) for changing the horizontal and vertical issue and the fonts issue . Now you can work on A BLANK TEMPLATE (woaaaaa, that would totally rock and it's like totally crazy!) and making thumbnails optional!