I highlighted these two comments and would counter with this statement--horses for course--depending on what sort of pavement and gravel roads you are on, for me this is the major deciding factor for type of tires.
My experience of 25 years of touring and regular commuting on good to reasonable paved and gravel roads has shown me that riding with narrower and less tank like tires can be perfectly fine. I have never had a sidewall failure in all my riding life, and have not ridden in goat head country. Riding on various kinds of roads, and using regular common sense in terms of not riding into sharp stuff that can damage a sidewall, the concerns here have never been an issue for me.
I know riders who I see regularly riding up against rocks and whatnot with the sides of their tires, who just are not spatially aware of how they ride close to sharp objects and lean into stuff from the side, so in a lot of riding situations, how you ride is a factor and how you hit stuff, unload your bike a bit if you do go into holes etc etc.
that said, if I were to ride loaded somewhere with much more dubious roads, and or much rougher/sharp/deep gravel, stones and dirt stuff, thats a whole other kettle of fish.
I bring this up because I figure the majority of people here tour in first world areas, on reasonable roads, in N America or Europe, and my experience of this has had no issues at all with lighter, less tank like tires. If going to South America or something, heck even doing more backroads here in Canada or the US, where you just dont know what sort of surfaces you'll be on, thats a diff story, but my experience goes against the fear of tire failures in a lot of conditions that I regularly ride in.
The thing is that I don't know what kind of roads we'll be facing. Europe has quite a bit of different possible road surfaces. There are roads in the Alps (which we might or might not ride) which would shred a road tire to pieces. Some of our local roads gravel roads are bad for tires no matter how good a rider you are. I do not want to bbe in a situation where I might have skip an interesting looking road because I wanted a lighter tire.
I won't regret the marathons even if we end up rideuing all of our tours on glass smooth tarmac (although I hope we won't. Some of the coolest stuff can be found at the end of a bad road)