I don’t care to belong to any club that will have me as a member.

The famed comedian Groucho Marx may have been kidding, but would investors feel the same way if they had to go through an approval process to use volatile ETFs? Leveraged funds, including those focused on single stocks, have exploded in number and popularity in recent years. But the products are so potentially dangerous in the wrong hands that Bloomberg’s editorial board suggested this week that brokerages should restrict access to customers who haven’t been vetted, and that strategies should be limited to 2x leverage. The op-ed piece followed disastrous episodes of South Korean leveraged ETFs crashing as chip-maker stocks fell. And it comes as the Securities and…