The biggest benefit is legal. Interactions between unions and management are governed by labor law. Many of the collectively-bargained elements of the NBA/NBPA relationship - the salary cap and NBA draft for example - are perfectly legal under that set of rules.
The relationship between the league and players in the absence of a union would be governed under anti-trust law. And that gives the players a wide range of options, including an anti-trust lawsuit.
On the other hand, decertifying also means the NBPA would be giving up a lot of benefits they've achieved through collective bargaining, such as minimum salaries, pensions and insurance. And the legal benefits are no sure thing; the league could argue that the NBPA is using decertification as a short-term negotiating tactic.