Sponsors, according to its thinking at the time, would have to more closely monitor the investments available through the brokerage windows in their plans. 

That would mean new fiduciary responsibilities for sponsors, potentially multiplying disclosure requirements and more. And that, of course, would mean new costs for sponsors to worry about.

Proponents of brokerage windows say they can spruce up the stalest of 401(k) investment menus. Through them, participants can access thousands of investment options. Large cap, small cap, cheap and expensive mutual funds, commodity funds — almost anything available to the retail market.

By design, using a brokerage window puts the onus on employees to develop their own investment strategy and do their own monitoring. In other words, sponsors offer brokerage windows without assuming any fiduciary duty. 

Also read: Brokerage window accounts grow despite criticism

The DOL's action threatened to change all of that. The brokerage industry responded aggressively. Led by the Securities Industry and Financial Markets Association, the DOL was accused not just of proposing questionable regulations, but of circumventing the statutory rule-making process, which allows for a comment period for industry to weigh in on proposed new regulations. 

It worked. The DOL pulled back. Then, last August, it sent out a request asking the industry for feedback on brokerage windows. Not surprisingly, it saw plenty of support, from the securities industry and others.

The recurring theme in many of the comments it received was that no further regulation or guidance was necessary. 

One anonymous plan sponsor with $5 billion in participant assets went so far as to tell the DOL that new fiduciary requirements would cause it to drop its brokerage window. 

Despite that reaction, it's unlikely the DOL will simply leave matters as they are.

The ERISA Industry Committee, which represents the country's largest plans, half of whom offer brokerage windows, said the DOL should, if it does anything at all, distinguish sponsors who offer the windows in conjunction with a traditional 401(k) from those who only offer the window. 

One of the DOL's concerns is that some sponsors offer a brokerage window only as a way of evading fiduciary obligations that come with a 401(k) plan.

Whether that's truly a problem and how big it might be tough is tough to say. 

In Charles Schwab's comments, the broker-dealer said it does offer a window-only arrangement, but it is usually to "micro" plans, described as having less than $1 million in assets. 

Jennifer Flodin, the defined contribution practice leader for Chicago-based Plan Sponsor Advisors, a division of Pavilion Advisory Group, said in her 20 years advising plan sponsors, she's never encountered a plan offering only a brokerage window. 

"We are fans of them, but only for the right demographic," said Flodin.

For the "right demographic" typically suggests individuals with higher incomes, investors who can more easily withstand a large loss in their portfolio.   

Critics of brokerage windows fear they provide open access to unsophisticated investors who end up chasing performance or dumping retirement assets into "hot stocks." 

Proponents say that's not something they see a lot.

Among the plans that offer brokerage windows in Pavillion's $65 billion portfolio, the average utilization rate is less than 5 percent of participants, Flodin said. That figure jumps considerably for sponsors of professional practices, like doctors groups, when often half of participants access the windows, she said. 

But Flodin, like other plan advisors BenefitsPro spoke with, said her sponsor-clients rarely, if ever, allow the purchase of individual securities in brokerage accounts. 

Support for brokerage windows within the industry isn't universal, of course.

Craig Morningstar, COO of Scottsdale-based Dynamic Wealth Advisors, a firm that provides fiduciary compliance services outside its own network of 50 or so RIAs, said he has real concerns about how brokerage windows are used and that they've eroded retirement savings for too many participants. 

But he's not ready to throw the baby out with the bathwater. The windows aren't broken, said Morningstar; they're just a little dirty. 

"Brokerage windows were created as a product solution, not a fiduciary solution," he said. 

And as advisors and sponsors are exposed to mounting fiduciary scrutiny, the time to reconsider broker windows' place in 401(k) plans is due, he said. 

Costs for administering these accounts are not only high, but they can expose sponsors to fiduciary liability when the expense is spread among participants who don't use them — a prohibited transaction under ERISA, says Morningstar. 

He also echoed a common concern over brokerage window transaction fees, which over time can add substantially to overall plan costs, potentially costing some investors good-sized chunks of potential retirement income. 

Fran Gillis, retirement plan manager for Dynamic Wealth, said that when his firm is hired to run an analysis of a plan's performance, brokerage accounts often underperform standard 401(k) returns, and sometimes cost up to five times as much in fees. 

"Over the course of a savings career, that can mean huge losses, especially for wealthier investors," Gillis said. 

What's worse, even "sophisticated" participants may be unaware of how fees on investments through the windows eat into their savings, because tracking these costs is not required, said Gillis.

"There is no excuse to not track performance in how participants' investments through windows are performing, and how they compare in cost. The technology to do so is there," he said. 

For all of their criticisms, neither Morningstar nor Gillis are in favor of removing brokerage windows from plans. But they do think better regulations could help drive down costs, ensure better-quality investment options in windows and help participants accumulate more savings.

Throughout the debate, self-directed brokerage accounts have been growing. T.D. Ameritrade, the online broker-dealer based in Omaha, reported a 40 percent year-over-year in 2014. 

On the other hand, all of the recent attention on the fiduciary rule may have sponsors thinking twice about adding a window. 

Scott Matheson, a senior director at Raleigh, N.C.-based CAPTRUST, said he's seen no discernable new interest in adding windows. 

About a fifth of the plans in CAPTRUST's account books offer them, though that jumps to 40 percent for plans with more than $500 million in assets. 

Attila Toth, co-founder of Portfolio Evaluations, a Warren, New Jersey-based RIA that advises on $40 billion in plans assets, said sponsors seem to be in a holding pattern with brokerage accounts, waiting for the DOL's guidance. 

Flodin, for one, said she's advising clients to wait to implement a new window until the regulatory landscape is clear. 

The comment period closed months ago, but the DOL has yet to say when or even if it will announce new rules for brokerage windows. 

Calls to the DOL for this article were not returned.