The ranks of actively managed exchange-traded bond funds are growing fast. But whether they deserve a spot in investors’ portfolios is still a question mark.

Late last year Fidelity Investments rolled out three actively managed bond ETFs that employ three of the firm's flagship bond mutual-fund strategies. Then last month, State Street Global Advisors teamed up with Jeffrey Gundlach 's DoubleLine Capital to offer an actively managed bond ETF, SPDR DoubleLine Total Return Tactical ETF .