You also may want to consider how this interacts with the stepped up basis of estates. If you never sell the stock and it passes to your heirs with your estate, under current tax law the basis will increase from the purchase price to the market price at the time of transfer.
In a comment, you proposed:
Thinking more deeply though, I am a little skeptical that it's a free lunch: Say I buy stock A (a computer manufacturer) at $100 which I intend to hold long term. It ends up falling to $80 and the robo-advisor sells it for tax loss harvesting, buying stock B (a similar computer manufacturer) as a replacement. So I benefit from realizing those losses. HOWEVER, say both stocks then rise by 50% over 3 years. At this point, selling B gives me more capital gains tax than if I had held A through the losses, since A's rise from 80 back to 100 would have been free for me since I purchased at 100.
And then later thought
Although thinking even more (sorry, thinking out loud here), I guess I still come out ahead on taxes since I was able to deduct the $20 loss on A against ordinary income, and while I pay extra capital gains on B, that's a lower tax rate. So the free lunch is
$20*[number of shares]*([my tax bracket] - [capital gains rates])
That's true. And in addition to that, if you never sell B, which continues to rise to $200 (was last at $120 after a 50% increase from $80), the basis steps up to $200 on transfer to your heirs.
Of course, your estate may have to pay a 40% tax on the $200 before transferring the shares to your heirs. So this isn't exactly a free lunch either. But you have to pay that 40% tax regardless of the form in which the money is held. Cash, real estate, stocks, whatever. Whether you have a large or small capital gain on the stock is irrelevant to the estate tax.
This type of planning may not matter to you personally, but it is another aspect of what wealth management can impact.