In her Cricket Australia blog, Lisa Sthalekar asks: are four days enough for The Ashes Test? She brings to bear several arguments, so it is really worth reading; but her basic answer is: no - four days makes it too easy to play for a draw.
One thing we do need to bear in mind is that this is actually a four-and-a-half day Test in "men's" terms, because 100 overs are being bowled each day, rather than ninety. And (I think?) they've actually bowled them all every day, even with the rain yesterday.
However, I think that a bigger part of the picture on this occasion is the six points. Although they provide a huge incentive to win, they also provide an even bigger incentive not to lose; and I am quite convinced that if you'd offered either captain a draw at the start of the contest, they'd have taken it; rather than risk going into the one-days having to win five of the six remaining matches.
The irony is that the six points were allocated to try to emphasize the primacy and importance of the Test. All they've done (so far!) is reduce it to a bit of a trundle.