Correct me if I'm wrong, but theme powers don't automatically get better at higher level. You only get the extra damage if you decide to swap your normal 11th or 21st level powers for that power, right?
You're wrong. The automatically granted powers scale with level. You power-swap to gain additional theme powers.
The automatically granted "freebie" theme powers, however, are still generally not very powerful, with one exception (Gladiator).
Poisoned Strike? 1[W]+5 poison damage is less than almost every Rogue/Ranger at-will, and the effect it grants is better than slow but somewhat worse than the target automatically granting CA.
It's a pretty good effect (but the monster has leeway to avoid it), but imagine if it were not a freebie: would you trade, say, Dazing Strike or Blunder for it? I wouldn't.