The spec map was the best part of that update.
A plane is not the same shininess all over. And most of them were a good amount shinier than how we portray them.
The reason that skinners have to do that is that when the entire plane is shiny, it looks ridiculous because the panel lines, extremely dirty parts etc. shine through the same as anything else. You can't capture really interesting nuances like the different reflectivenesses of different paints used on the aircraft. The mattness that preceded them was fine, but with spec mapping you can make planes look a bit spiffier for sure.
Bump mapping and spec mapping are both central in taking your skins to that 'next level', in my opinion.
Unfortunately I got largely out of the game by the time spec maps were introduced, and only ended up playing around with them for a bit, but they're a really amazing tool for skinners to take advantage of, even on planes that don't include bare metal areas.
Also you can do most of the work by for both of them by just taking your original skin, converting it to grayscale, and playing with the colors/saturations of the layers
Also if you modified your panel lines at all from the original and don't change the bump or spec maps it will look stupid
Also if the default spec map takes in dirt/paint differences that aren't in your skin it will look stupid