My current finances are 18k in 401k(have only been working in civilian world for 1 1/2 years), and 17k in ROTH IRA's, and I estimate(because TSP sucks to attempt to regain access to) ~12k in TSP. Going forward, I'll be maxing pre-tax 401k contributions every year that I can, and maxing my ROTH IRA too. However, prior decisions and not trusting the age requirements behind traditional retirement vehicles has left me with 70k in a taxable account.
I'm all for pre-tax 401k contributions, as it's a lot easier to max that then to max Roth 401k contributions at my income level, and like the others said, most Mustachians expect low expenses/low taxes in retirement. The regular ROTH IRA however is just too good to pass up, especially with the 5 year waiting flexibility that you gain on the principal, which means I'm only 2 years away from starting to gain access to that money again.