"At the participant level, there's a large amount of inertia when it comes to Roth," said Matthew Cann, an ERISA consultant in the Vanguard Strategic Retirement Consulting unit.

Wendy Tyson, a senior ERISA consultant in the same unit, said that sponsors are taking their cues from the lack of participant interest, which is related to a number of factors.

"From a participant level, you have the baseline inertia around Roth conversion," she said. "In addition, a Roth in-plan conversion is a taxable event for participants — and … because the feature is not tied to a distributable event in the plan, they need to have that money outside the plan to pay that tax. And market fluctuations also come into play. In-plan conversions can't be undone. Participants don't want to pay taxes on a $20,000 Roth conversion that might only be worth $15,000 at tax time because the market has gone down."

Still, the IRS' release in December of guidance designed to help sponsors decide whether adopting Roth in-plan conversions is right for them may spur additional adoption, according to Tyson.

"One important thing the IRS made clear in its guidance is that there is an extended amendment period for ATRA," Tyson said. "Sponsors now have until the end of 2014 to adopt both Roth and the Roth in-plan conversion feature for their plan."

As for participants, Tyson and Cann agree it comes down to a personal decision. "We are Roth advocates to our plan-sponsor clients," Cann said. "But Roth in-plan conversions are an individual tax matter, and it's very hard to make general statements like 'it's the right thing for everyone' or 'it's not the right thing for everyone.' People have to understand their own tax situations, and perhaps consult a tax advisor, to see if it's right for them."