No, disk brakes are definitely not a fad. They are far more prevalent on mountain oriented bikes than road bikes. They appear on mountain bikes because they stop you faster -- especially in the wet. They do not appear on high end road bikes because those bikes are mostly concerned about decreasing weight in order to increase speed. Disks may offer superior braking performance -- but they add weight.
The Trek FX line is wierd becasue the orientation of the bikes change as you progress up the ladder...
The 7.2 has nice wide 700x35 tires with an aluminum frame and what Shimano classifies as a mountain bike derailer. The 7.7 has 700x25 tires with a carbon frame with all road bike components. They are very different bikes that will feel and handle much differently.
But, all of the bikes in that line are road bikes that are adapted to be more comfort and have a wider range of use. So you don't see much in the way of disk brakes in that line.
Conversely, the Trek DS line is primarily a mountain bike that can be used on the road. In that line you see far more disk brakes. Actually, the only ones that don't are at the low end of the line: the 8.1 & 8.2 have rim brakes. The rest (8.3, 8.4, 8.5, 8.6) all have disk brakes that progress from mechanical to hydraulic.
Perhaps the main reason for investing in disk brakes is their stopping power in the wet. If you expect to be riding in the rain or snow, then disks will do a better job for you. Otherwise, rim brakes can work well enough and save some weight.
The question is more about what do you want? A fast little sports car or an all-terrain Jeep? They are both good -- but each is better at some things than others. The hybrids (like the FX line) tend to split the difference between road and mountain.