Maybe it's important that eBay is "real time" in the last 5 minutes of an auction, but the rest of the time, the vast majority of the content is relatively static. A seller might update the description of a listing a couple times over a two week auction, for example. And while it sounds great to immediately update my search results when a new listing goes live, in reality, I already have 40 pages of results to look through, and that listing that just went live 5 seconds ago probably isn't much more relevant than any of the others I'm sifting through.
I'm not opposed to client-heavy apps where it makes sense. When done well, it can create a really responsive user experience. Gmail is great at this; I have no desire for it to be "real time" -- not any more than it already is.
Do we really believe that one day cnn.com will be "real-time", with article updates and errata popping up inline as we read?