Sure, it can be annoying when the players oneshot your boss,
And this is my main concern with this rule. You know... there is the climatic final battle against the Orc shaman, and then, in the first round, the warrior of the party gets close to the Big Bad and scores a critical hit and he spends his action die... and then he (a soldier with STR 18) rolls maximum damage with his Zweihander...... All of that in the first round.
Yeah, yeah, I know, I'm over-exaggerating the probability of that scenario happening, but the probability is still there.... I don't know... Can the GM spend his own action dice to "cancel" the action dice spent by the players?
And then again, why are critical misses so mild when compared to critical hits? I mean, a critical hit is "probability of K.O.ing the Big bad in the first round", while critical misses are "probability of suffering a setback that, at worse, should only delay the players in the completion of their quest". Shouldn't critical hits be more like critical misses? Maybe critical hits should only grant the players useful and flavorful advantages or bonuses for the rest of the combat, or something like that.
We had a fight in our game where I rolled up to the Bad with my drake MA and double-critted him (note: bite + MA feats = 16+ threats, ouchies!) into a fine red mist. On the flip of that the drake has received at least two crits that nearly dropped him in one hit. GM turned on the NPC auto-activate threat CQ and the big dragon gets a lot of attention...
So, it plays both ways. If you want to have less spikey combat, turn crits to vitality instead of wounds with a bonus to dmg or possibly a free roll on the Table of Ouch.