I'll take a stab at that. Here are my thoughts on why a Roth sometimes outweighs a regular IRA for MMMs:
Better for long term savings.
A Roths growth will never be taxed. Assuming the stock market grows at 7%, once you've kept your money in there for 8 to 10 years (for the 25% and 28% brackets respectively), you're overcoming the initial higher taxes via non-taxed growth (assuming you slip down to the 15% bracket)
Extra savings option
Traditional IRAs are not available to people already offered an employer plan (401k, 403b, etc.). The Roth allows you to squirrel away an extra sum.
Much greater flexibility
Contributions can be withdrawn tax and penalty free for ANY reason. Traditional IRA's have many more restrictions.
You'll need some Roth money to get started
If you're planning on utilizing some sort of Roth conversion strategy during the MMM phase, you'd need at least five times that yearly sum already in your Roth when you enter retirement.
Protection against future tax increases
Consensus seems to be that there's no where to go but up with the tax rates. Relying on the aforementioned rollover strategy leaves you vulnerable to tax law changes. Hell, they could even do away with Roth conversions at some point.
Self-Life-Insured?
There also might be an argument that this acts as good life insurance, especially for those with children, as, I believe, the money passed on to anyone would be tax free. Once the stache is big enough, you're basically self-insured. But I'm too young to be an expert on the estate side of things, so I may be wrong?
Not subject to Required Minimum Distributions
Again, I'm not an expert on the later in life stages of investing but I could see how being told you must withdrawal x amount from your plan every year could mess with your ERE math.
Finally, one of my biggest reasons for building Roths is that I just think it's generally wise to have a mix of types of assets available in ERE. So, those are my reasons--I'm really not an expert though! And I'll be giving more thought to how/why a conversion strategy might be a better option for me.