I think the weakspots on certain Vex units are a sign of malformed construction, almost intentional because they are throwaway replaceable units. With bosses and the like, having a critical point at least to the Vex seems counter productive to the unit's success. I like to imagine that weakpoints only occur when you actually "weaken" the enemy, by stunning, cornering, knocking over. That kind of thing. I actually like the way you have to down Crota, and can only damage him via the sword. That kind of weakspot is more interesting to me. The unknown to be discovered as it were.

I wouldn't call that a crit spot, it doesn't deal any extra damage, just stops them from teleporting.

That pisses me off every time I see one