Most of these postings are pure theory. In fact you almost never need to use friction mode because your pannier pretty much protect your rear derailleur if the bike flops over while stopped and crashing a fully loaded bike is unusual.

Even if you screw up your rear derailleur exactly where in the world are you going to be where Shimano won't be the most common deraiileur?

I like and prefer the FEEL of bar end shifters and I can tell by the feel what gear I'm in. I can EASE the shift so that there isn't a loud CLACK when I shift. And as Michel Gagnon pointed out, if you have a handlebar bag (and what tourers don't?) combined shifters may not work properly though that's a bit of a stretch.

The real advantage is that they're less than half the price of a combined shifter, a whole lot less complicated and almost never fail. I've replaced about a half dozen failed Ultegra levers and a couple failed Campy levers though they were the lowest end stuff from a long time ago.