In effect, they are an extra benefit sponsors can use to remain competitive in the talent attraction and retention game.

But SDBAs are not without their critics.

The Department of Labor recently issued a request for information on the accounts, or "brokerage windows," a forerunner in the process of issuing new guidance.

Previous RFIs on the accounts yielded substantial concerns.

Many told the DOL that brokerage windows present unnecessary risk for participants, because fiduciaries are not engaged in the review of the investments selected. That means participants may be exposed to expensive fees on some investments—something sponsors are trying to avoid these days.

Others have told the DOL that in some cases, the same investments made from a brokerage window can be made in a traditional menu line-up, and much more cheaply.

Critics also say there is no real mechanism in place to allow access only to investors sophisticated enough to know what they are getting in to. Using high dollar thresholds to limit access as a way to determine who is knowledgeable enough to assume the risk of a brokerage window brings up nondiscrimination issues.

While brokers have the ability to do so, they rarely limit the options available. That means even the most expensive, opaque and risky instruments can be made available. And ultimately, the cost of administering the window is shared with the participants in a plan who choose not to access it, say critics of SDBAs.

Greg Carpenter, founder and CEO of Employee Fiduciary, an advisor to the small plan market, is staunchly against SDBAs.

"Brokerage windows undermine the fiduciary process and devalue the beneficial work done by fiduciaries," wrote Carpenter in a blog for BenefitsPro. "What seems like increased choice is actually a rejection of the fiduciary process. Bottom line: the increased choice often results in a net loss versus a fiduciary solution."

Trading costs at the windows ultimately eat into too much of a participant's savings, Carpenter feels.

A bi-weekly contribution of $700 to a single investment, at $10 per trade, comes to $260 in trading costs, or about 1.4 percent of the contribution.

"Add additional investments and sell trades and the costs can go much higher," he wrote.

While the chorus of critics appears substantial, interest in SDBAs is growing at TD Ameritrade, according to Gary Major, director of the self-directed plan services business unit there.

The Omaha, Neb.-based broker saw a 40 percent year-over-year increase in the accounts, said Major.

"With respect to our accounts, only 3 to 5 percent of participants with access to the windows actually use them," he explained.

"If the windows are accessed by an unusually high number of participants, that indicates that something is wrong with the core 401(k) line-up," he added.

The low rate of participation in brokerage windows is a fact critics don't often include in their arguments. And according to Major, it's not as if those who are accessing the windows are pouring cash into vague investments.

"We find people typically are investing in blue chip stocks, well-known mutual funds, and ETFs that offer a lower expense ratio than mutual funds in a lot of core menu line-ups," said Major.

The bottom-line in Major's opinion: "We feel very strongly that the brokerage window is a valuable way to provide investment choice and flexibility to plan participants looking for that."