You can sift through EMG studies and find that a very high EMG study on a muscle is often correlated with a better exercise. For example, if I do exercise A and compare it to exercise B in regards to abdominal activity and exercise A is higher, exercise A is regarded as better.
There was a study done long ago that showed that squats/deadlifts had very high EMG scores in the abdominals. Hence the advice, "just deadlift and squat and your core strength is fine."
One of the things I've taken away from the "Lateralizations and Regressions" DVD by Charlie Weingroff is that perhaps the fitness of person is different. If two people are squatting and one person's EMG is through the roof in the abs and the other person's are not, both squatting same speed/weight, the person with less EMG is in better shape, they have more left in the tank.
"EMG doesn't lie, but it doesn't tell the whole truth." Charlie Weingroff.
So EMG isn't the definition of better exercise.