BOSTON ( TheStreet) -- Among the consequences of the recent Great Recession were the flaws exposed in many target date funds.The mutual funds were billed as a "set it and leave it" approach for retirement savings, built around an end date with investments that would switch from stocks to bonds as the years passed. Ideally, the funds simplify asset allocation and rebalancing while being immune to the whims of individual investors who might make rash, damaging decisions based on short-term stock-market movements. Many of those with 2010 funds learned the hard way that the basic premise wasn't always followed, finding themselves with a larger-than-expected allocation to stocks that proved disastrous as the Dow fell by more than half in the months following Lehman's bankruptcy in September 2008. According to the Securities and Exchange Commission, funds with a target date of 2010 averaged 24% in losses in 2008. Hardest hit was the Oppenheimer Transition 2010 fund, which shed 41% of its value. In 2009, returns continued to vary for 2010 target date funds, ranging between gains of 7% and 31%. Despite concerns, the popularity of target date funds continues to surge. Assets of target date funds registered with the SEC total $270 billion, up from $256 billion at the end of 2009 and $160 billion in December 2008. Hewitt Associates' ( HEW) annual study of company benefits showed that premixed portfolios, including target date and target-risk funds, now make up the largest portion of employees' asset allocations. The jump can be largely attributed to increases in the number of employers using the funds as the investment default under automatic enrollment. Hewitt's research shows that among the 58% of employers automatically enrolling workers in 401(k) plans, 69% assigned them a target date fund. Nevertheless, there are drawbacks to the "set it and forget it" approach. Because the funds are actively managed, they have higher fees those of index funds. Risk tolerance isn't customized by the one-size-fits-all strategy. And for investors who pick their own funds, there are plenty of mistakes to be made. One financial adviser recently commiserated about a client whose portfolio consisted of multiple target date funds in a misguided attempt to be diversified. The point of target date funds is that just one can satisfy investors' needs.