In some cases, their silence helps US too. I just sold my iPad 2 by simply posting on Facebook that I was selling it to raise money to pay for the next one. Within 15 minutes the deal was done, but I got about 5 responses from Facebook friends saying things like, "There's a new one coming out? When?" and "What is supposed to be on the new one?"
Apple staying mum on future products keeps the current ones selling. The only people that know new stuff is coming out are the tech blog watchers like us. Most other people pay no attention.