Oh my! I do love that apogee of XS development, the 1977 "D"---stock of course, but mine was the blue rather than the maroon. Both are beautiful. Now mine is black and gold, with a temporary tank from a '75.
Why keep them stock? I'm tempted to be an intelligent rectum and say: "Brains"....but that's not really fair. So let's see:
Modified bikes:
1. increase the value of my stock ones
2. provide a source of stock parts
3. make their owners happy, at least for short rides.
4. provide an experimental base for genuine improvements, electrical for example
5. keep makers of weld-on hard-tails in business.....
.....and there may be others. I dunno. My bike isn't stock. It has bits from 1975, 1977 and 1979, but it looks like what it is: a 1977 D" with 63,000 miles on her. I love her, I ride the bejaysus out of her, and she gets washed when it rains....
Ummm....what was the question again?