Jason Cohen:

90% utilization is causing more failure than you realize, not just in burn-out, but in productivity and output.

…

Worse, in many organizations everyone is operating at 90%, which then reacts like the three-server system, where the inevitable hiccup from any one person causes a ripple effect that hurts several other people or projects. Since they are over capacity, rather than absorb the spike, they too will ripple the problem to others—a cascade like the the run-away chain reaction of an atom bomb.