[tptacek]

You want to risk pool a specific cohort of people. You want the pool to be as large as possible without masking clear adverse indicators. For instance, from an example downthread: you probably don't want to pool people with trampolines in with everybody else. Most people don't have trampolines. To an insurer, the sole function of a trampoline is to generate lawsuits. If you pool trampoliners together with everybody else, you necessarily raise everybody's rates to subsidize trampoline lawsuits. Better to factor the trampolines out and price them directly.