The biggest downside is the limited amount of contributions you and your husband are allowed to make each year. So, if you are sitting on a pile of cash, you can each contribution $5,500 to your 2013 Roth IRA before April 15, 2014. And you can put in another $5,500 each for 2014 before April 15, 2015. That's $22,000 sitting in your Roth IRA invested which can sit there and earn money and can be withdrawn tax-free whenever you need the money. Say you pull it out to buy a car. Yes, you've earned tax free gains on $11,000-$22,000 for about two years, which isn't much in the grand scheme of things. Or, you can leave it in the Roth IRA and allow it to earn for 10+ years, which will be more significant, especially if you continue to contribute to your IRAs.
The biggest downside to your plan is that you cannot put the money back in! Each year, you are limited to $5,500 (may go up in the future, but not by much). So if you put in $5,500 each in 2013 and 2014, then pull all $22,000 in 2015 leaving only the gains, you can only put about $5,500 back in for 2015. You've put money into a retirement account to save for a car, withdrawn the amount for the car, and then you can never put that money back in because of the annual limits.
I think if you were going to spend that money on a car regardless of whether it is in a savings account or an IRA, sure, put it in your Roth IRA and withdraw your contributions. You would have the advantages of earning tax-free gains. However, realize the effect that this will have on your retirement plans, if that is something you are starting to plan for. The ideal plan (if you have enough cash for it), is to put $5,500 into each of your Roth IRAs for 2013 now (before April 15), save for a car in a savings account, while also setting aside another $5,500 each for 2014 IRA contributions.