I've been out of cycling for 6-7 years and have come back feeling a bit confused about tubeless. Having done a bit of web searching, I have come to some conclusions about this.
The issues of resistance to forward motion related to wheels are rotational mass - the lower- the better the acceleration for any given force/torque
"Gravitational resistance" - ie, mass of the wheel as a whole up a hill.
Other attributes include puncture resistance, comfort (annular pneumatic suspension in other words) and friction in providing centripetal force in cornering.
I have not seen a convincing argument supporting tubeless compared to clincher/tube relating to weight.
Rolling resistance is a very small part of overall resistance especially above 20kph, where aerodynamics come to the fore. Then , the biggest aerodynamic issue by far is the position of the rider on the bike as opposed to the profile of the wheel.
Indeed Sir Chrs Hoy recommends attention to proper positioning on the bike to extract greatest power for minimum loss on the bike ahead of new carbon wheels. Wise words.
Rolling resistance is largely caused by deformation of the tyre. A wider tyre reduces rolling resistance compared to narrower tyre for the same pressure, but at higher speeds this could be at the expense of aerodynamics unless married to a particular rim design.
It is said by some (see earlier posts) that a lower PSI will reduce rolling resistance and this may the case on rough tracks and lower speeds as seen in MTB, but on tarmac this is less likely to be the case. I believe some are confusing tubeless rolling resistance being lower, with a wider tyre size, which does in fact reduce rolling resistance.
Tubeless has the advantage of reduced snakebites or pinch flats and as this is the case, a lower pressure provides a more comfortable ride as the annular pneumatic suspension is more compliant, but for the same width of tyre, one with a higher pressure on tarmac will have a lower rolling resistance.
Check out Scwalbe's tech info on their website for reference on this.
So my opinion on this, is that tubeless would be great for training and long sportives. It certainly is the standard in MTB. However for the current cost premium of a dedicated tubeless set of road wheels, I'd rather spend my money elsewhere and get clinchers.