I totally agree with keeping only a few dozen books on a Kindle. I see no need other than ego to put several thousand books on an eInk Kindle.

First, there is no way a person will read several thousand books before the Kindle dies of old age.

Second, the Collections feature is so limited that you would have trouble finding a particular book with that many on the Kindle.

Third, you couldn't use the kindle for a research tool, not with that many books. A lot of grad students might be tempted to try, but the Kindle simply isn't fast enough, powerful enough, or have the right feature set to work with a vast library of books as far as research goes.

Fourth, the indexing is brutal with that many books and will slow the Kindle down to a crawl.