Your ECC system is substantially more expensive; it's not just the RAM. $50 more for comparable RAM, $100-$150 more for a board capable of supporting it, and $100-$150 more for slower CPUs. It's the total cost of the platform, not just the RAM, that causes a sticking point. $250 or $300 goes a long way in performance. ECC is all of a sudden the difference between a single $200 card and a pair, and it doesn't provide any tangible benefit to the guy who wanted to game.
5-10% is important once it represents the difference between playable and not. You're absolutely right that the difference between 110 and 120fps on a 60hz LCD is pointless and comical, but mid-range systems don't tend to do that. If you can keep it at 60-70fps on today's games, then next year's start dropping a little below that point. I'll bet the person asking for a gaming computer will want every performance gain he can get to keep him in the playable range for the next few years.
Random crashes? I hate to fight your strawman with anecdotal evidence, but it's not really a problem. The flipped bit has to be one in use at the time it flips, and it has to be one that's actually important (and not just changing color on a texture or something). I haven't used ECC since the EDO RAM era, and I've never had a problem with system instability other than in the dark days of Windows 9x.
My best may be a webpage that's a little vague (who benchmarks products without trying to keep things as similar as possible?), but you haven't even gone that far. Seems as if you're sticking to handwaving for this one. Your argument has boiled down to two points thus far:
A) UPTIME! Besides months of uptime being useless to someone gaming (due to frequent reboots for Windows patches), I hope you understand that there isn't some mass "crash fest" afoot for the majority of the users not running ECC. Seriously, your system's uptime and stability, like any other well-built computer, likely owes far more to a good PSU, reputable brand components, and good cooling than it does ECC.
B) It's good enough; you might pay a bit more and get a little less performance, but that's not such a big deal. Except that it is a big deal, because every dollar you advocate spending on hardware with minimal benefit is a dollar not spent on hardware with an actual tangible benefit.
You and I both know that the best you could honestly tell someone in this position is "It might one day keep your OS or game from crashing, but I can't really tell you the likelihood that it will."