When exchange-traded funds (ETFs) burst onto the investment scene more than two decades ago, one of the initial selling points of the asset class was intraday liquidity. Whereas mutual funds trade at only one price throughout the trading day, ETFs trade in similar fashion to equities, in many cases with constantly changing prices. This advantage also turned into a criticism, as ETF naysayers alleged that the asset class would turn ordinary investors into day traders. Unfortunately for ETF critics, data indicate that investors are buying and holding ETFs.

"As ETFs have become more popular holdings, the average ETF was held for 26 months in advisory accounts, while it was 19 months for retail accounts. Digging more deeply, investors were even more patient with U.S. and international equity ETFs than taxable fixed income ones," said CFRA Research, citing data from fund giant Fidelity, in a note out Monday. (See also: Top 3 ETFs for Long-Term Investors.)

Given the recent growth trajectory of ETFs, particularly in the U.S., the world's largest ETF market, it is hard to argue that investors are flipping in and out of ETFs in rapid-fire fashion. Investors have added money to U.S. ETFs in 12 consecutive months. At the end of February, exchange-traded products (ETPs) including ETFs listed on global exchanges had a combined $3.84 trillion in assets under management, Investopedia reported earlier this month.

While intraday liquidity is a valid selling point when it comes to ETFs, the factor that is more prominent in terms of driving money to ETFs is fees. ETFs usually have significantly lower fees than actively managed mutual funds – and that matters to buy-and-hold investors. (See also: 3 Ways to Avoid High ETF Fees.)

"The average holding period for U.S. equity and international equity ETFs was 27 months and 25 months in advisory accounts, respectively, and 20 months for both categories for retail accounts," said CFRA. "To CFRA, this confirms to us that such low-cost products such as iShares Core S&P 500 (IVV) and iShares Core MSCI EAFE (IEFA) are increasingly replacing mutual funds that were used for long-term asset allocation needs."

In further proof that fees matter, IVV earlier this month became just the second U.S.-listed ETF to top $100 billion in assets under management. IVV charges just 0.04 percent per year, or $4 on a $10,000 investment, making it one of the least expensive broad-market ETFs in the U.S. (See also: Another ETF Joins the $100B Club.)