I think, if you have light bike you can make more kilometers a day then you touring
I agree very much!
Asking blankly for the "lightest touring bike" is a bit hard though, as what constitutes touring varies a huge amount, and type of roads travelled, climate, logistical constraints (ie, do you have to cycle hundreds of km at a time between water sources and food, or can you just get away with a small bike bottle and buy food for each meal because the place is so densely populated), weight and size of rider, and weight and volume of equipment carried, your ability to maintain the said bike, are all some of the decision which factor into what bike is an appropriate touring bike for you.
I have seen some people tour, however, with some pretty light set ups. In the future, barring financial restrictions, I could definitely see myself touring solo unsupported and camping on the way, on a bike that without gear weighs close to the 6.8kg UCI limit. If I didn't care about throwing bikes away every tour, I'm sure I could go lower than that. Much of my ability to do so is down to my skills and experience touring in a lot of countries around the world, and ultralight hiking and camping. For example, I envisage I would have my gear load (not inc food and water) down to below 3kg, which is almost no additional strain on the bike frame, wheels and components. However, this set up would only be for relatively mild conditions (eg European summer, most of Australia in winter except the Alps) and if I was say, travelling in Africa, I'd have to add weight to both my gear and bike; at least 1kg to both, probably more.
Definitely the biggest inhibitor in my experience to people riding lightweight touring bikes in the realm of road racing bike weights is the weight of gear and food and water carried. Its not abnormal for a typical touring cyclist to have 20kg (or more) of gear strapped to their bike in four panniers plus a handlebar bag and a rack top bag, before they even add any food and water. While a road bike doesn't have to be that much stronger for an 80kg cyclist vs a 60kg cyclist, there is a big difference between a 60kg cyclist with 0kg of gear and a 60kg cyclist with 20kg of gear. The gear is static, not centred between the wheels, and can't ease itself over bumps and shift its weight around appropriately. The bike is now taking strain in a large number of directions, just just through the pedals and handlebars (and a little through the seat if you are sitting down) over bumps.
Igor Kovske has a great website on how he has done some ultralight touring.
Also see Peter's article on crazyguyonabike.com ...he did some relatively ultralight touring quite cheaply using an old road bike.
For getting your gear weight down (not inc Bike maintenance and tools) the best place on the WWW to go is backpackinglight.com