Will insurers avoid the ACA's exchanges?

An insurer can choose not to offer any products in the exchanges--the do nothing scenario... It could also be prudent to stay out of the exchanges for a year or two, and then join and offer products after the dust has settled.  

From a Milliman analysis of "Ten critical considerations for health insurance plans evaluating participation in public exchange markets."  Consideration #10, on why plans might want to stay out of the exchanges, is of particular interest.  It mentions the possibility that the exchanges will attract "a population with unknown, and potentially unfavorable, risk characteristics."  This scenario would likely drive health populations away from the exchanges and lead to adverse selection within the exchange, a big worry for ACA supporters.