If you earn over a certain amount, the Roth IRA option isn’t available to you, you’re left to go after the Traditional IRA. If you participate in a 401(k), you can’t even deduct the contributions to a Traditional IRA. So, why would anyone ever contribute to a non-deductible Traditional IRA? If you did, you’d be paying taxes on the contributions and taxes on the disbursements when you retire… that’s double taxing and that’s foolish! However, there is one reason why you would want to do this. Conversion!
Right now, only those who earned fewer than $100,000 a year can convert a Traditional IRA to a Roth IRA. However, the Tax Increase Prevention and Reconciliation Act of 2005, signed in May 2006, introduced a conversion loophole in 2010 that removed that $100,000 rule. In 2010, anyone is permitted to convert from a Traditional IRA to a Roth IRA. What this also means is that it gives those above the Roth IRA income phaseout to have a back-door method of contributing to their Roth IRA.
When you convert a Traditional IRA to a Roth IRA, you pay taxes on the sum because you were able to deduct the contributions. If you made contributions to a non-deductible Traditional IRA, you already paid the tax so you can make the conversion absolutely free. That’s the only reason I can see for contributing to a non-deductible Traditional IRA… as long as they don’t close the loophole.