ETFs are the fastest-growing segment of the money-management business. While popular in DB plans, ETFs in 401(k)s are still infrequent, typically available only in those that offer participants a "self-directed" option. Still, their place in 401(k) plans has grown over the past couple of years as plan sponsors begin to better understand the benefits of including them on plan menus.

In 2010, TD Ameritrade created what it called the first ever commission-free ETF. The funds, hand-selected by Morningstar, were made available to retail investors. 

Since then brokers like Charles Schwab, and Fidelity, which markets certain iShares ETFs commission-free, are pitching the cost-advantage of commission-free ETFs as a way to attract more RIA business.

Several days ago, a top Pershing executive told Investment News that her firm also is considering a commission-free trading platform for ETF.

"We're actively looking at that as a further potential solution, evaluating many things including the economics of the program," said Sandy Bolton, managing director of financial solutions at Pershing. "We want to ensure when we launch a program like that, that it is a robust platform."

Proponents of ETFs say they give RIAs a new way to help sponsors, and participants, diversify their portfolios, and to do so cheaply.

According to PwC, ETFs now account for approximately $1.8 trillion in assets under management domestically. BlackRock is the industry leader, with almost $1 trillion in ETF assets. Other big players include Vanguard Group and State Street Corp.

Most ETFs are comprised of U.S.-based equities, but more commonly they're including commodities, and other non-traditional investments like foreign equity exchanges, giving advisors more diversification options without the risk that comes with picking a single security.

And when those investments can be purchased commission-free, RIAs, who are compensated on a fee-basis, can move sponsor clients into TDFs without incurring costs and having to pass them onto clients — a competitive advantage more RIAs are taking advantage of.

Broker-dealers are being forced to take notice.

Next week, Florida-based independent broker-dealer Raymond James will roll out access to a commission-free ETF trading platform, available only to its associated RIAs, according to etf.com.

Raymond James' RIA unit includes about 110 RIA firms and more than $10 billion in assets, according to Reuters.

However, unlike some of the others companies, Raymond James plans to focus more on actively managed ETFs instead of the passively managed ETFs that largely constitute the broader ETF market. 

In September, Schwab expanded its line-up of available commission-free ETFs to include 182 funds covering 65 investment categories. The San Francisco-based broker permits access to both retail clients, and to the 7,000 independent RIAs that use Schwab as the custodian for their clients. 

The effort to lure RIA business with commission-free trades through Schwab's OneSource platform seems to be working. At the end of August, the platform had $31 billion in assets. 

Schwab's overall ETF assets amounted to about $216 billion at the end of the third quarter, up 20 percent from the previous year. The firm's RIA channel accounts for 51 percent of those assets. 

More than half of Schwab's new ETF inflows this year have gone into the firm's commission-free OneSource platform, according to Schwab. 

Clearly, the movement to compress fees in retirement plans is encouraging the overall ETF movement. 

And the rapid growth of commission-free ETFs is driving more brokers to offer them. 

Which begs the questions: how soon before we see ETFs turn the corner with defined contribution plans?