Why always the maximum?
Shouldn't it be the average value for the hit die, as the fixed amount? Otherwise groups that do use random roll are always going to be weaker overall, which - depending how finely-tuned the math is - could throw a monkey wrench into adventure design.
I'd think average would be the way to go.
I'm not sure how they're doing monster hit points right now, so this may be way off. I'm going to use 3rd edition as an example. Third edition monsters used the average of their hit dice to determine their hp in the monster manuals. Some examples:
Troglodyte 2d8+4 (13hp)
Ankheg 3d10+12 (28hp)
Great Wyrm black dragon 37d12+296 (536hp)
We've seen the math for hit point distribution. It approximates a bell curve as the number of levels goes up. Thus, player hit points will tend toward average over the course of a campaign. The makers of 3E assumed players had average hit points.
Were a DM to simply grant maximum hit points to his players, monsters would need to similarly be given maximum hit points as well in order to maintain their challenge to the group. So, in a game with maximum hit points, that Great Wyrm dragon should have 713 hit points.
Then, we run into one of the criticisms of 4E: monsters have too many hit points, making encounters run too long. It'll also throw off the expected healing, making healing spells and hit dice half as effective over time, on average.
Like I said, the monsters may be using a different method for hit point generation than the players (and it seems like they are, but it's impossible to know how it's done without those rules released to us). Still, for testing, it makes more sense to assume the average and base the balance around that. It is impossible to balance around the entire spectrum of possible hit point totals, making average hit points the most simple method.
That is why I'd like to see static hit points as the default, with rolled hit points, max hit points and whatever else presented as options.
I'd also like to see the current rules for natural healing (or something like them) remain as the default, with "gritty" and "4e" style health modules added in as options.