ETFs also help to prevent an influx of hot money going into long-term funds, where it could disrupt and drive up transaction costs for long-term shareholders.

"They act as pretty good safety valves, especially for these less-liquid asset classes" such as the high-yield bonds held in the Total Return fund, said Michael Temple, director of credit research at Boston-based Pioneer Investments. Rather than money coming out of open-ended bond mutual funds, "the 'hot money' tends to be in ETFs."

To be sure, ETFs can spark volatility in corners of the bond market, too, as they foster frequent trading, putting them at odds with long-term investors, according to analysts. Hedge funds, for example, have made big, quick moves in and out of junk-bond ETFs, creating a ripple of volatility for the market to absorb.

Opportunity Knocks

BlackRock's iShares ETF may have been the biggest beneficiary of last week's Pimco selloff - it had $404 million in new money entering the fund on Monday in its biggest single-day gain this year. On Tuesday, the fund added another $349 million.

Vanguard’s Total Bond Market ETF also spiked on Monday and Tuesday, albeit in smaller amounts, as investors looked for places to invest their money amid the fallout at Pimco. The fund is up 0.56 percent since Friday.

The iShares iBoxx $ Investment Grade Corporate Bond ETF for corporate exposure and the iShares 1-3 Year Treasury Bond ETF for short duration exposure are also liquid funds that are good spots for temporary money, Nadig said. LQD is up 0.70 percent and SHY has risen 0.13 percent since Friday.

Without the availability of ETFs, fast-trading investors might not have such broad portfolios to jump into and out of. Vanguard, for example, blocks short-term investors from using traditional bond mutual funds aimed at long-term investors. It is common for traditional mutual funds to charge investors who sell quickly after they buy.

"Clearly there's some reallocation going on in the market place," as some money moves out of long-term funds into ETFs, said Greg Davis, global bond chief at Vanguard Group.

Meanwhile, those temporary moves may not be lightning fast. Those holding periods can be anywhere from a couple of days to several months, said BlackRock's Tucker. Even if the money ultimately leaves BlackRock, every $1 billion left in the ETF with a 0.08 percent annual expense ratio can leave the asset management company about $26.7 million richer in just four months. And some money may turn out to be far less temporary than expected, if investors like EP Wealth's Ashworth decide to stop chasing active management and instead embrace ETF's index-following, low-cost approach.