As far as I can tell, the main benefit of jailbreaking is to be able to install a reader that supports more ebook formats, since a stock Kindle can already browse the web and read ebooks from any source, but it only supports MOBI/AZW3, and the Send to Kindle EPUB converter is not very good. But it’s easy enough to use Calibre to do a lossless EPUB -> AZW3 conversion without the hassle of jailbreaking. Is there some other benefit that I’m missing?
It’s a bit more than just supporting additional formats. KOReader for example is a highly customizable (and mature) reader that’s hard to beat. Plenty of plugins exist also. In particular, one nice thing about the Calibre plugin is wireless sending of books and progress sync across device(s) and desktop.