eirk That's the point. You encounter them occasionally but infrequently enough that browsers must allow this. Often ebooks are encoded in this format. They are also similar to PDFs, so I see no harm in providing support. Also, from wikipedia:
The advantage of DjVu is that it is possible to take a high-resolution scan (300–400 DPI), good enough for both on-screen reading and printing, and store it very efficiently. Several dozens of 300 DPI black-and-white scans can be stored in less than a megabyte.