An other crack at explaining...
First, you have to divide these bikes into two categories:
Category one is bike like the R1150GS and the Suzuki V-Strom. They are never intended to really go off road. They are comfortable, easy to ride bikes that will handle a dirt or gravel road and work incredibly well on twisty, narrow back roads. They will never match a super sport bike's time on a race track but in the real word they do very well. A 110 x 19 front tire puts about the same amount of rubber on the road as a 120 x 17 but the long, narrow foot print provides a very stable, reassuring feel. Add the high, wide bars and you get a bike that is easy on bad backs (the long distance to the pegs is easy on bad knees as well) and can still be made to turn quickly because of the increased leverage. Also, the "Adventure Tourer" name and styling makes them much more palatable than an old, boring "Standard" which is what they really are.
The second category are bike like the old (R80 and R100) GS BMWs, KLR650s and the KTMs. These bikes are identified by 21 inch front wheels and much less weight. Even the very porky (relative to the group) post '90 R100GS was close 500 pounds ready to ride after removing some of the non-essential but government required junk. While not dirt bikes they will take a rider (and a tent, food, sleeping gear and such) in relative comfort to a distant dirt road and then further down that road and a single track trail off that road than people who haven't tried them would believe. You will not be doing any triple jumps or flying over whoops but a successful stream crossing on bike this size can be pretty rewarding. I can report that an R100GS heavily loaded with camping gear and a large rider will indicate about 90 MPH across a dry lake bed and provide a large thrill to the rider.
And yes, just like every type of bike a fair number of adventure tourers are purchased by posers who will never use anywhere near the full potential of the bike. BMW's new R1150 Adventurer is pretty much a Range Rover with two wheels and is, I believe, aimed directly at these people. The bigger bags and extra fuel are nice but knobbies on a six hundred + pound bike? Pretty much the same thing as guy standing next to a 996SPS with no wear on the tires within an inch of the edge of the tread or the "biker" on his $30K Harley who has never ridden more than 25 miles from his four thousand square foot, tri-level executive home in the gated community and couldn't find the sparkplugs without help.
I'm hoping the new KTM will set new standards in the second category, we'll see.
