No! Bad Dm's! No Rules! Bad!
I think the focus on the underlying math and the way of retro-engineering monsters is the wrong kind of "think". Specifically, one of the main things I am looking forward to in the new edition is being able to look at a bunch of critters of an approximate power level and then intuit a new monster based on a gestalt of the others.
"Hmmm...these roles at this power level give around this many hp, attack, and defense bonuses. Now let's add in a funky power or two on par with these others or just swipe a power from this other critter. Figure out why he's there and what he's doing. Done."
As opposed to "Ok, now I take this concrete number and crunch it against this and we have exactly 7.5 hp per whatever with a static bonus of 4.3 per whatever. Now let's add in this other variable and tweak the whole thing yet again. Almost there. Next we add this template. Just a minute. Be right there. Advance the hit dice. Carry the 2."
The second example is specifically what the designers are moving AWAY from. They don't intend for creature creation to be 3.x anymore and I couldn't be happier. General guides based on their hard work is good enough for me. I don't WANT specifics because then my whiny players will expect me to USE specifics.
Ballparking not only keeps down the prep time but increases player suspense. Players should not be retro-engineering critters in the middle of a fight to determine strategy. And yes, I DO have players that do that, despite my best efforts to avoid it.
Keeping it loose allows more flexibility in the encounters. Players tearing through the critters too fast? Add a "when bloodied" nastiness on the fly. Players getting trounced? Reduce the critter's hp or defense a touch.
And the players never have to know. All they see is "Wow. I didn't think we would pull through" or "Jeez. That was tougher than I thought it would be." Not all the time, but enough to keep the suspense.