It's not about opening up safari or chrome. HTML5 "apps" are simply a browser window and the guts are implemented in HTML/Javascript/CSS. This is how LinkedIn's app used to work and it is the basis for Phonegap (which powers most HTML5 based apps).

The memory management bit Prasad references is a big deal too. Can you imagine not being able to release a feature that you've invested a lot of money into because you've got a performance problem or crashing issue that the tooling is not good enough to let you solve?