Before I answer your question, I'd like to make a general observation about this thread. I've read all the posts and I see that a lot of pro-tower answers go on about other things besides the enclosure itself. Perhaps it would be better to refer to a specific model of the speakers which differ only in the enclosure (floorstander vs. bookshelf). There are many such speakers where a bookshelf has it's exact match in floorstander as if a bookshelf was popped into a bigger box. This would focus the debate to one difference that OP put in his question. Something like; all other things being exactly the same, what would you benefit from getting a tower speaker. (OK, I know all other things are not the same, if the box is bigger, f3 is tuned differently and the x-over is never exactly the same, but let's say that's the minimum of differences).
I'm saying this bc there ARE very well made, genuine, 3way bookshelf speakers, so number of the drivers is not the main issue. Let's talk mostly about the volume of the enclosure and what it brings to the table.
@storing Few things do come to mind, but I would immediately agree that it has to do with the quality of the product as well. Having said that, I don't think it's all about SPL. A small-smallish room of hard walls might give such a bump to your low region that you have to attenuate it almost all the way with some speakers that have an otherwise good low region. So saying 40% (it's a number from the top of my head only to serve the point in question) might refer to other properties of your speaker. It would mean you bought a speaker that does exceptionally well in the 40-60 range only to have to take it all the way down.
Of course, it's mostly practicality and cost, but then again every overkill is. Still, there IS such a thing as overkill.