So the obvious question (well at least for me!) is why not just run the tubeless liquid in a tubed tyre if it seals good? Looks like many are already doing this:
well the main reason is that when you get a puncture, the thorn/nail/whatever sometimes leaves the sharp pointy bit actually
in the tyre casing.
so the modern sealant (liquid latex stuff) will seal the tube. But remember that the tube and the tyre are individual items that move separate to each other. So yep... the sealant seals the tube. And the thorn/pointy bit, well it just pokes a new hole a couple of mm or so away from the original hole, then again another few mm away and so on.
Anyone who's changed a std tube and forgotten to check the tyre properly for the thorn and gotten 50m up the road while the new tube deflates will know what I mean.
For general cyclists and run of the mill family bikes... I would recommend thornproof tubes with slime. And if possible tyre liners on HD puncture resistant tyres too. (that's what I used to commute on)
When I was commuting regularly... I ran an internally geared rear hub, dynamo front hub, 29x2.0" Schwalbe Marathon Supreme touring tyres with puncture resistant casing PLUS some old Mr Tuffy tyre liners and thronproof tubes... with Slime in the tubes. Clearly weight was a not an issue or a concern. Getting to work on time without stopping on the side of the M4 or M7 was...
For off road riding on a decent mtb... tubeless all the way.
It's not about weight (though that is a minor bonus) it's about reliability.