Small businesses would be able to join together in co-operatives to buy health insurance under a Tennessee bill, but critics doubt whether it would lead to significantly lower costs or more coverage for uninsured workers. The small-business groups likely to form wouldn't be large enough to attract most insurers, detractors say. But proponents say that the improved stability and predictability of rates that comes with larger groups would help ease the burden on small employers when trying to offer affordable health insurance benefits.