Imaro
Legend
How often do those big wins or crushing defeats happen? Every session? Every other? Once a month, year, decade of play? How often DO PCs face something non-level appropriate? Once a session? Once a year?
It reminds me of the argument for rolling hp vs. static hp/level. Lots of people claim the ability to roll a 10 on a d10 outweighs the risk of rolling a 1. Still, most people through the course of 20 levels end up with hp close to the average, barring an odd string of good luck or bad luck. At the end of the day though, hp seems to average out to around where the static amount would be anyway.
Is a level-appropriate tailored campaign where most PCs face level appropriate challenge (+/- 4) really all that different than a game where you sometimes fight kobolds, sometimes fight hill giants, but mostly end up fighting level-appropriate challenges? In the end of the day, aren't you more likely to face a level-approprate challenge than not?
Let me answer your question like this... all it takes is one amazing win or accomplishment to have a story for life... literally, players will talk about this type of win forever. Of course if they're never given the oportunity to at least try it if they want to.... Not to mention, IMO, it gives the world a more realistic feel...YMMV of course.
Basically I feel like you're trying to numerically codify something that can't really be codified that way, at least not for those who enjoy this type of thing.
EDIT: And in the end, how often is totally up to the PC's to decide, not me as the DM.