I'd love to see some stats on this. I wonder just what percentage of users never changes a default UI.
It will depend a lot on the specific product. For example, I'm sure that Eclipse very rarely runs with fully default settings. Consumer products will run in more of a default state, until they get very important to people's lives (I've noticed an increasing awareness and ability with Facebook privacy settings among non-technical friends in the last year).
My parents definitely won't.
Well, that's at least 2.8741 × 10^(-8) percent of users!
What's your logic behind this number? Do you think Microsoft made all those billions selling crappy software to geeks like us?
I'd wager it's 2 / CURRENT_EARTH_POP / 100.
GP asked for stats, I supplied the best stats I could infer from the information you provided.