> Which is exactly why it should be on by default.
>
> If it's invisible unless users already know about it, then most users
> won't benefit from it.

The users that care will, but even that's beside the point.

> Privacy is generally expected to be the default, it is then up to you
> to decide what you want to tell others about yourself.

Microsoft itself, which used to set DNT by default, blogged about why a DNT harder should not be set by default here: http://blogs.microsoft.com/on-the-issues/2015/04/03/an-update-on-microsofts-approach-to-do-not-track/

If you're right that networks ignore it anyway then setting it achieves nothing, but adds data about the user for networks to track. If you're wrong, and some networks do honour it, you're undermining it's purpose by setting it by default.

Educate your users about how they can take advantage of the features you provide to safeguard their own privacy; don't presume to make decisions for them.

--
"Racing turtles, the grapefruit is winning..."