You can't look at the math in a vacuum. If you are commonly seeing powers that can add +4 defense here, subtract -4 attack there, add this protection, vulnerability, etc you can not make conclusions about the system without seriously taking this into account.
I'll give you guys an example from my group. Two of my players both have a power that gives an enemy a -7 to their attack roll! That's a pretty large impact right there.
So without solid inclusion of this in the math analysis, I would fall back to playtest experience to see what is going on.
The question comes up:
How many powers does the average group have that give a bonus to hit or a bonus to defense or penalizes the opponents, AND, how long do those powers last?
It's great to give an enemy a -7 to their attack roll. If it is only a single attack roll, then 1 encounter in 3 (assuming it is an encounter power), that power does something to compensate for the math problem. 2 encounters in 3, that power does nothing to compensate for the math problem.
If it is a daily power and does not last for multiple rounds, then it helps one attack per 3 days.
So, one cannot just assume that the synergies of powers compensate for the math issue that people are definitely aware of.
One thing we do know (due to the math problem):
At level 30, there is a penalty to hit of 4 on every single attack.
At level 30, there is (typically) a penalty to the best defense of 4 on every single attack against that defense type.
At level 30, there is (typically) a penalty to the medium defense of 4 on every single attack.
At level 30, there is (typically) a penalty to the medium defense of 7 on every single attack.
So as a rough estimate, 100% of PC attacks are at -4 and 44% (non-AC attacks in the MM) of monster attacks are at -5 (on average) at that level. Shy of feats or Paragon Path abililties to overcome this by 1 or 2 pre-PHB II feats.
Are there really that many +x powers that compensate on average for all attacks made by the PCs and nearly half of the attacks made by the monsters for an entire encounter? To my knowledge, most bonuses and penalties are not for the entire encounter. They tend to be to the end of the next turn.
When one considers that most high level encounters last for many turns (often easily 15 to 20+), a power that lasts until the end of the next turn lasts for a very short portion of the encounter.
Ditto for powers that require a "to hit" in order to gain a bonus (e.g. Lance of Faith or Righteous Brand). The fact that they hit less often means that they also give their bonus less often.
So yes, it is important to consider powers. And powers, especially powers that create conditions, do help the party. But, monsters have powers too. Monsters can have auras or attacks that penalize the party or stun them or whatever. They just typically do not have as many such powers.
One cannot just assume that the synergies of powers make up for -1 PC to hit and -1 PC defenses (not AC) of each tier.