One of the problems with putting out patch notes ahead of time is this:
Lowered drop rate of item xxx.
...and everyone's going to go farm the item before the patch hits.

Added the following recipe:
...and everyone's going to stock up on ingredients, killing the economy for said item

Eliminated the sp gain for people that mass spam-heal while never killing a mob.
...and everyone's going to go exploit this "feature" before it gets patched.

Added new r50 weapon that makes old r50 weapon obsolete
...and it effectively makes the "old" r50 weapon obsolete already.

You get the idea.