Out in the real world, the average young person probably has good eyesight and the average middle aged person has so-so eyesight and older people generally have crappy eyesight.
But then there are lots of people of all ages who have poor vision, weird quirks, colour blindness, tunnel vision, or clinical blindness.
Arbitrarily reducing a font from small to teeny is basically a slap in the face to anyone with less than perfect eyesight.
Yes, a user can adjust their browser settings to enlarge the fonts but when you do that to Facebook, for example, it makes all the text larger and the whole thing looks like shite.
Notably my 2 issues were:
1) My eyes hurt so much that I preferred using my iPhone over a 24" screen. After installing this, my eyes are feeling much better. Maybe I'm just old.
2) Before, my Friends' status messages were 13px, everything else was 11px. I prefer that my friends' thoughts be more important than UI chrome.
