You need to say the same thing over and over to refine your own understanding and presentation. You need to do this not just once, but periodically thereafter, because the thing you believed in 2007 will be different when you restate it in 2010. Experience has taught you things since then.
You need to say the same thing over and over because students don't learn it all the first time. They, too, grow in experience over time. When I reread the things that I read twenty years ago, I see completely different nuances than I did before.
But, most importantly, you need to say the same thing over and over because most people didn't hear you the first time, and even if they did, the world keeps making more people. If nobody ever reposts or retweets the classics, they rapidly become forgotten. (And then they aren't classics anymore: The reason classics are classics is that people can't stop quoting them.)
We've "all" heard that NDAs are worthless and that execution matters... and yet prospective clients keep pulling out those crazy NDAs. We've all heard that agile development is better than waterfall... except for the thousands of programmers who are working on waterfall projects right now. We all know that there is No Silver Bullet, and yet silver bullets continue to appear and continue to be touted, almost 25 years after the original essay.
It's okay that Spolsky himself no longer feels called to the vocation of patiently teaching the same things, over and over, to new generations of programmers and entrepreneurs. That job is not for everyone, and he's got a few other things he could be doing. ;) And it's also true that the web does a miserable job of segregating the newbs from the old hands, such that it's hard to avoid hearing lessons repeated more times than you really want to. But however much Spolsky may not need to write the 18,001st restatement of No Silver Bullet, there are always going to be people who need to read it. We're never going to finish the job of teaching people. They aren't born knowing.