I would put it down to there finally being a distinction between web-sites and web-apps.

Within an app, my current context (which control has the focus etc) is important and a full page reload loses all of that.

Within a web site, as it is less interactive, this stuff doesn't matter so much.

As to whether New Twitter is a site or an app is debatable (I say site and therefore shouldn't be using #!). And as for Gawker...