I can think of a few reasons, mostly aesthetic, like "Because the mixes look good on the shelf" or "Because it gives me a greater sense of achievement" or "Because it's more fun/interesting".
But from a more practical viewpoint, how about these?
1. Maybe you have a better setup, but I still have problems with reliably digitizing things. The original mix of my new album sounds like it's on vinyl, and not in a good way. Lots of clicks and skips, especially if I try to digitize under Windows, which frequently goes off to do something of its own instead of servicing the audio interrupts. In that situation, I can go back and re-digitize from the original tape. A mix direct-to-disk and I'd have to remix the whole thing if I later found clicks in it. Of course, I have also had the converse, tape dropouts and so forth, but if you're using a proper 3-head deck you'll hear those as they occur or just afterwards, on playback.
2. If and when standards move past 24/96 or 24/192, you can go back and digitize the tape again to the new standard du jour. Or, heck, say you score a new machine with better converters.
3. Cataloguing. I don't know about you but I have a great problem holding onto digital files. Sometimes I've run out of space and deleted things which I've later regretted, but most of the time I simply have so much stuff that I can't keep track of it all. Maybe you have more output than me, but it's fairly quick and easy for me to pull the tapes off the shelf and glance at the names on the back. Searching all the filesystems for all WAV files and then trying to find out which one is the final take of "Recipe for Disasters" is a problem.
4. Having the end product in as many different formats as possible increases the likelihood it can be recovered years down the line.
Actually, I'd be interested to know how you're managing backup if you're doing an all-digital setup.
Other issue: No Firewire card!!!