Absolutely with you. I thought this is just one of those really poorly conceived powers. If it was for one turn with the full bonus, fine. If it was a much smaller bonus till end of combat, fine. But a massive (read "make you all but unhittable") till end of combat? Does ANY game (not Just D&D, ANY game) ever designed have space for such a thing?The answer is yes, this is a poorly designed power. In the epic game I ran, the party sorcerer was about as tough to kill as the party warden, via a few key items and utility powers (as compared to an entire dedicated build).
When they nerfed "Lead the Attack" (which had the same problem) my hope was that they realized that linking a d20 roll to a stat was dangerous because stats scaled up to the point that probabilities could reach the 5-10%/90-95% end of the spectrum (i.e. a total balance fubar) and therefore should never be till end of combat.
Precedent dint follow through though. Ah well.
My take is that anything linked to hit points (damage, DR, healing, temps) can scale hard as you level, but things linked to a d20 should not scale at all (I keep saying this over and over, a +1 bonus on a d20 at level 1 is just as potent as a +1 bonus at level 30).