Although the Pension Protection Act of 2006 didn't allow stable value funds to be selected as a qualified default investment alternative, 58 percent of boomers have allocations to stable value funds in their defined-contribution plan. Nearly one-third of Gen Y investors and 46 percent of Gen X investors have some portion of their DC fund allocated to stable value funds.

"A principal preservation asset class was once an afterthought for most sponsors and advisors, with a recordkeeper's proprietary solution being an easy choice," Patrick Murphy, managing director of sales for New York Life Retirement Plan Services, said in a statement. "No longer. Our advisors know this core asset class demands the same level of scrutiny as any other investment option."

Murphy noted that this trend was likely to continue. "With interest rates expected to be negligible for the foreseeable future, money markets are not a palatable option for most sponsors – and stable value investments need to deliver a competitive rate of return with a credible guarantee." 

Reuters reported on Wednesday that stable value funds may not remain stable. Over the past few years, the insurers who guarantee the principle in stable value funds have limited the types of investments they're willing to cover.

J.P. Morgan Asset Management and Pacific Investment Management Co no longer guarantee principal on stable value funds they offer, Reuters reports, and have made it more difficult to access funds quickly. Some 401(k) plans use funds that only guarantee a portion of the principal.

"It's a stark contrast from the stable value ethos that has existed for the better part of four decades, leaving some industry observers wondering if the new iteration of the funds is worth the investment," Reuters reports. "The trend, if it continues, could mean less flexibility for plan sponsors and fewer guarantees for investors."