My reasons for doing this were (YMMV)
1. Leaving the Roth money in my account would estimate to have over $220k of gains by the time I was 59.5 I would not be able to withdraw any of that money prior to that and could cause me to have to withdraw more from my traditional money and could cause me to be in a higher tax bracket then I needed to be.
2. If you are married and make less then $101k then your capital gains and dividends are already taxed at 0% so the only benefit of the Roth would be the same in the taxable account in this situation (you would need to look at your state tax rates to make sure they were not too high to be a material amount)*
3. With a taxable account, you can tax loss harvest which will save you additional taxes and can make it more tax efficient than the roth (you can also tax gain harvest to lock in your 0% rates now)
4. I am not real confident that the government will not start taxing withdraws from Roth IRA's sometime down the line as well.
- I think if you are not able to fully max out your 401k or traditional IRA one year then this is a no-brainer to take the contributions and put it into your 401k/trad IRA
And if you need the gains as well, I suppose you could just pay the 10% (penalty) tax. Its still lower than the 15% long term capital gains rate.
To withdraw this you would have to pay the 10% penalty plus your tax rate even though it is Roth money.
* You would need to be investing in stocks. This would change if you do not have enough space for your bond allocation.