The folks at the Investment Company Institute, the trade organization for mutual fund companies, recently released some interesting numbers. It seems that some 44% of households in America, nearly 51 million of them, own shares of mutual funds. Is this a significant increase or decrease over last year's numbers? Uh ... no, it's not. The ownership level has pretty much held steady.

To me, though, that's noteworthy -- because what we should see is a strong jump in ownership. We should want to see 100% ownership, or something close to that. Why? Because for anyone planning for retirement, investing in mutual funds is incredibly valuable. Here are some reasons why.

You want to retire
First off, nearly all of us need to face the fact that we have to save for our retirement and other financial needs. We can't rely solely on Social Security, decreasing numbers of us have pensions coming to us, and few of us have amassed sufficiently hefty nest eggs to live on later.

Next, mutual funds offer a hard-to-beat combination of features: While many of them are overpriced underperformers, there are still lots of terrific funds that don't charge too much, that beat the overall market in most years, and that have talented managers at their helms, making smart investment decisions for us, the shareholders. With a single investment in a mutual fund, you can immediately be invested in a diversified group of stocks and/or bonds.

You can, of course, tackle investing on your own. Many do so, and do well at it over the long haul. But it's important to recognize that not everyone is cut out to study stocks and their corresponding annual reports and financial statements. Many of us don't have the time or interest for that, and many others don't have the skills. I myself have recently been funneling more and more of my money into mutual funds. Why kid myself that I'm a savvier investor than, say, Peter Doyle and Murray Stahl, who run Kinetics Paradigm (FUND:WWNPX), with its five-year average annual return of more than 26%? Plenty of my picks haven't done as well as theirs, which include Leucadia (NYSE:LUK), Reliant Energy (NYSE:RRI), China Life (NYSE:LFC), and IntercontinentalExchange (NYSE:ICE).

Given that there are plenty of talented managers out there, why aren't more of us tapping their services?

Retirement matters
One noticeable change in mutual fund ownership in America in recent years is the growing influence of tax-deferred retirement accounts, such as 401(k) plans and IRAs. According to the study, "45.4 million households own funds through ... tax-deferred accounts, up from 35.7 million in 1998." That's good, but it's still not enough. According to a recent Fidelity Investments study, only about two-thirds of employees take advantage of their work-sponsored retirement plans.

You need to be actively building a hefty nest egg for your retirement. Take advantage of tax-deferred plans available to you, and consider doing even more than that. I encourage you to test-drive, via a free 30-day trial, our Rule Your Retirement newsletter service. It distills what you really need to know into a manageable volume each month. A free trial will give you full access to all past issues, allowing you to gather valuable tips and even read how some folks have retired early and well. It regularly offers recommendations of promising stocks and mutual funds, too.

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Longtime Fool contributor Selena Maranjian does not own shares of any companies mentioned in this article. Try any of our investing services free for 30 days. The Motley Fool is Fools writing for Fools.