They're definitely fantastic marketers. Everyone repeats the line about tax-loss harvesting.
The truth is that the excess after-tax performance of tax-loss harvesting will stop helping you after a short period of time.
And you can do it yourself, too.
How do the benefits of tax-loss harvesting stop after a short period of time ?
If you're investing with the expectation that your money will grow, you must also acknowledge that Tax Loss Harvesting on any one particular deposit will eventually not be possible (there will be no losses to harvest).
What about dividend re-investment or additional funds ? Won't my year 2 contributions also be eligible for tax-loss harvesting ?
Yes, your year 2 contributions will be eligible, but that won't help your year 1 contributions. If you keep making bigger and bigger contributions, you can mask the fact that your earlier contributions have lost more in fees than they've gained in harvesting, but that doesn't change the underlying numbers, and eventually it will catch up to you.
It is inevitable, for the simple reason that the fees are both percentage based (so they get higher as your account grows), and forever (each and every year, for the rest of your life), while the tax loss harvesting benefit on each individual deposit is temporary.
I get what you are saying - TLH ( actual tax savings ) are limited to $3000 per year ( and future roll-overs are limited to X years you are alive). As you dump more $$ into your account the fees also increase and TLH is limited to the recent deposits.
But also consider that tax-loss harvesting also wipes out long-term and short term capital gains - would this be useful when rebalancing ?
If you only rebalance once a year ( in 15 minutes between stocks & bonds ), you don't see this benefit.
But if you are spread out between 11 asset classes ( or individual stocks in WealthFront 500k) couldn't you take advantage for example of commodities going down 5% one month, do some TLH out and rebalancing over to international developed, then sell some long term bonds and buy US mid-cap. Keeping your portfolio balanced and not incurring any taxable events ?
Could a machine algorithm optimize that several times over the course of a year without any transaction costs ?
It's even worse than you're describing:
1. You won't get the full $3,000 every year. From what I've seen from researching the stocks in Betterment's portfolio,
all tax loss harvesting activity ceases for any particular deposit, after a year or so, and sometimes less. This is a chart of all of Betterment's stocks:

They all hit their bottom in the 6th month. A theoretical Betterment portfolio which started 4 years ago, would have had 0 tax loss harvesting after the
6th month. While the portfolio, and the fee, would have nearly doubled. It is unlikely this money will ever have the opportunity to tax loss harvest again. It will end up looking like this growth chart of the S&P500 (red line):

Maybe 2010 was a bad year to start, lets try some others:
2011:
1.5 years
2012:
1 year
2009:
3 months
2004-2007 all hit their bottom at the predictable 2008-2009 bottom, which was easily captured with manual tax loss harvesting. So between 1-3 years was the maximum tax-loss-harvesting time.
2003:
3 months
2002:
9 months
...etc
As you can see, after the first year or so, tax loss harvesting ceases. Your portfolio, however, rises significantly, along with the percentage-based fee.
2. You can't simply say, "Oh look, commodities are down 5% this month, let's tax loss harvest some of that." You can't tax loss harvest, unless it has fallen
lower than your purchase point. If you bought in at $100, and the fund is up 10% since you bought it ($110), you can't tax loss harvest if it then falls down to $105, or even $100. It has to fall
lower than $100. If it falls to $95, and you do tax loss harvesting at that point, this lowers your cost basis (the price you got in at) to $95. Now price has to fall lower than $95 in order to perform further tax loss harvesting.
So it's easy to see, that if you're investing with the expectation that your money will rise, tax loss harvesting has a limited shelf life.
Regarding the rebalancing question, rebalancing simply isn't that big of a deal. No one should be paying any transaction costs with buying/selling, as you should be in diversified index funds, which don't have transaction costs. Also, Taxable events when rebalancing, really don't matter if you either:
1. Contribute new money regularly, allowing you to rebalance with that (you're in the accumulation phase).
2. Withdraw money regularly, allowing you to rebalance with that (you're in the distribution phase).
or
3. Have a 401k, or better yet an IRA, where you can rebalance without worrying about taxes.