These have no fans. No moving parts to worry about. That is one of their biggest advantages. I can use an ARM netbook while my SO is sleeping next to me. Not so for an actual laptop.
For anything other than gaming, I don't even notice my Air has a fan (I agree it's -very- noisy when it's running, but the point is for the stuff you're going to do on a Chromebook the fan isn't going to be noticeable on any other laptop either), and that's the same for most modern laptops I've used.
I brought up the Air mostly because I didn't know what the noise was when he started the game. I thought that the Air didn't have any moving parts. In fact, I'm still debating getting one, but a machine that's silent at 250 that can do what I want it to is a much faster decision than one that is ~1000.
I know my use case isn't typical, but it is the one I rely on when deciding what to get for myself, and silent is good for my needs, and how much noise the computer makes when I've got all cores running on a compile is a factor, for your needs it may not be, or you may not do the same type of work.
The point I'm getting at is that no matter what I do on a machine with no moving parts, it's going to be silent. An older laptop on eBay may be more useful for some people, and I'll never fully move away from a workhorse machine until ARM performance is on par, but it's still nice to have when she's sleeping and I'm still mulling over a fix.
That said, I think you're significantly overstating the noise of a modern Mac in the first place. Some of the non-retina MBPs run pretty warn, but my last-gen Air was silent unless I tried to play games on it. Even when writing code. I could build a ~500 file Java project in IntelliJ and it wouldn't even get warm, let alone kick on the fans.
You said yourself that you're not in a position to fully change to something ARM-based because of performance issues, and that's precisely my point: what's the point of saying the Chromebook is superior in it's silence? I might as well say that a piece of paper has superior battery life to my laptop because I can use it for 24 hours without charging. They do different things.
(On a tangent, what are the options for actually developing software using ChromeOS, my understanding of it was there was little to no access to local filesystems for that sort of thing? Or are you talking about installing some other OS on it?
Edit: Read your post a bit further down about putting Gentoo on it. Still curious if there's much scope for developing in ChromeOS itself... did a bit of searching but not a lot of the hits sounded practical or were native.)
That is a ridiculous statement. My mac air is silent under normal use for everything but 3d gaming.