I love it in principle but, considering how small most books are relatively to storage space offered by modern devices, even mobile ones, what is the upside of storing content centrally instead of just carrying a copy with you?
Also a content server mitigates you having to manage and synchronise your ebooks in multiple places. You only need to manage one library. Although I guess any sync application can work just as well for this.
I can also throw up another instance to serve to specific groups, like relatives or kids, so they can download and network print deemed safe content like crosswords or recipes.
You can probably see why I wouldn't want that on my phone all at once.
https://github.com/seblucas/cops https://wiki.mobileread.com/wiki/OPDS
Also a single central location + backup ensures that your books are unlikely to be lost.