Why hullo thar money making scheme to drag the cash cow out a bit more!
Seriously, why couldn't they have made the interesting, plotty, exciting books (Goblet Of Fire and Order Of The Phoenix) into two films instead? Do we really need an hour of them camping, being useless and emoing at each other? Couldn't they cover most of those 20 odd chapters in a montage?
And where exactly do they end the first part - that rambling mid section isn't exactly going to make for an exciting first film finale. Even if you split it up you have to ensure that each film has a distinct beginning, middle and end (see Fellowship of the Ring and Two Towers for examples of that well done but remember they are individual books). I'm worried this could all end up being distinctly crappy.
Still at least they haven't done the same with Half Blood Prince (which I'm sure will make a great film since there's more than ample padding in that book that can be removed without touching the story).