Why do I want all the books I have on my iPad? Because I can. Why do I keep all the t-shirts I own in my closet and not in a box in the garage? Because I have the space in my closet. I have the space on my iPad too.
The same reason I have 8,000 songs (yes, all legally purchased between myself and my wife with a few purchased off iTunes from my daughter). Why you may ask? Because I like all sorts of music, from rap to rock, from classical to show tunes, to famous speeches. Any one catagory may have only 20-30 files, rock has thousands, classical has hundreds, rap has maybe 50. People who question why remind me of the people who questioned why anyone would ever need more than a 40mb hard drive, then they questioned why you would ever want more than a 500mb hard drive because is was HUGE!! And now here we are where you can easily have over a terrabyte in your laptop (no, I am not quite there yet, I have 800gb in my laptop, between a 160gb C: drive and another 640gb in my second bay). My iTunes folder is easily over 200gb.
So why you ask? Because I can. And because I travel for a living, and am rarely at home, so I like to keep it all with me. My iPad is used by my whole family as well, so I have books on there for everyone, just as I have music on my iPod that appeals to everyone.
But seriously, enough about why, riddle me this: Why would Apple limit how many books I can keep on my iPad. Memory is cheap, at the size of the files you are talking about it is essentially free. As more and more poeple purchase and use their iPad as reference devices I am sure my paltry collection will look small in comparison. Imagine the computer repair tech who needs thousands of reference sources to maintain a diverse use base, and also wants to keep his continuing education files as well. There are easily dozens if not hundreds of user type who want more access to more data. To limit it is to limit their market. It would be like Amazon saying "Buy this 64gb kindle, but you can only load 2gb of books on it or it becomes unstable" How stupid would that be?