The density of AP 2 also varies greatly by metagame and opponent.
Some lists, like the Necrons for example, have a lot of trouble shoveling lots of AP2 into their lists. Others, like the Imperial Guard, can put it everywhere and anywhere.
Most armies I've faced will have 1-2 units that can kick out substantial amounts of plasma fire. Using a terminator-heavy army, then, simply becomes an issue of taking out or neutralizing those specific threats.
Then there's the Eldar and the Imperial Guard--good luck with that.