I've been fascinated by this phenomenon for a long time.
The best coders I've found are not the loudest voices. In many cases they do not comment publicly on the web at all. They just submit bug fixes.
That said, the best code I've found is the stuff that's made publicly available via internet, and I'd argue it's precisely because it allows all those "silent experts" to review it and submit their fixes. It's not the best simply because of the contributions of the silent experts, but because their scrutiny raises the bar.
It takes a certain amount of confidence to make code avaiable for all to see. Because it becomes very clear, quite quickly, whether it's any good or not.
I don't see this same type of peer review and vetting within the corporation. The goal there is different: one need only satisfy the company's clients, most of whom cannot themselves review code quality.
In my experience, many of the silent experts who submit fixes work at corporations. They are no strangers to closed source.
So it's all quite interesting. Open source and open discussion in its own strange way effectively raises the bar. But at the same time the web also lowers the bar to spreading bad code. And disseminating much nonsense chatter about programming.
Hence we have lots of chaff. Lots and lots of chaff. The wheat is there, but you have to know where to look. And generally, when you find wheat, you'll also find lots of "silent experts" submitting fixes.
I often wish which they were the ones making the noise on the web. Because then it wouldn't be noise. Many of these guys really know their stuff. I guess over time the web has worsened things. In the early days, many online contributors, e.g., to bulletin boards and newsgroups, really were knowledgeable in their chosen fields. There were just less people online. That seems to have changed somewhat, but maybe it's just my perception.
In any event, they might be silent, but we know they are paying attention. No one is going to completely ignore the continuing growth of open source and open collaboration.