https://www.tsp.gov/...oth/index.shtml
My thoughts (other investment vehicles not considered):
I dump $5k into my Roth IRA. In an ideal world, this $5k comes from tax free sources such as CZTE pay or left over TDY money so that it is never taxed. My next goal is $17k into the TSP. In the past (and until they finally open the Roth TSP), this contribution would serve to reduce my taxable income. If my ability to contribute is limited to $17k (i.e. I am unable to take advantage of the CZTE additional limit of $50k), there is only marginal advantage to putting tax free money into the Traditional TSP (this advantage being that the contribution will never be taxed). However, there is significant advantage to using tax free money in a Roth TSP.
So, let's say I wind up with a six month CAOC tour later this year after the Roth TSP is available (assume May-Oct deployment for shits and giggles). Let's also say I'm terrible at managing my tax free trips and don't manage a cha-ching for the remaining six months. Also, let's say I have everything set up so that I contribute monthly to both my IRA and TSP ($416.67 and $1416.67 respectively). Since I'm deployed so long, I plan to max out my SDP as well.
So, at the end of April I have $1666.68 in my Roth IRA and $5666.68 in the Traditional TSP. Before I deploy, I change my TSP allocations to the Roth TSP (same account) effective 1 May. I continue contributing throughout the deployment in this manner and prior to rotating home I change my allocations back to 100% into the Traditional TSP. So my balances at the end of the year are Roth IRA: $5000 (of which $2500.02 is never taxed and $2500.98 is post-tax); Traditional TSP: $8500 (all tax deferred); Roth TSP: $8500 (of which the entire amount is never taxed); SDP: $10000.
That is how I envision the Roth TSP being useful to most of us...sheltering tax free money for life.