frankthedm,
Its not really feasible to cut HP in half and double the monsters. Monsters can be bloodied after one hit in some cases and this wreaks havoc with some of the designs of monsters. Some powers only work when bloodied, others work when not bloodied. Additionally, some monster roles require othr monster roles to exist for them to be functional. If you can quickly eliminate one role, the other monster becomes far less effective and essentially ruins the monster role balance for 4e.
At least this is what we have found in the heroic tier. Maybe its different in the paragon and epic tiers.