Is meritocracy ideal? Is a purely meritocracy society the one that maximizes happiness for the greatest number of people?
Assuming we discover a perfect way to measure merit, what do we do with people who lack merit? What alternatives do you offer them over crime or suicide?
I'd be concerned about problems from having a sharper divide between "employable" and "not employable", and instead of people who struggle to find/keep a job you'd have (slightly fewer) people who can't get jobs at all. That seems likely to cause more social problems.