Sure. But I suppose not every file needs to be re-done, so why not build an installer that just replaces/adds the necessary bmaps/bgls?
In FSX, the photoreal background bgl takes about 80% of the whole installer space because it contains textures inside the BGL. That one will surely change when additional runways and their related taxiways will be added so, there wouldn't be much saving comparing to the full installer. And, since the photoreal scenery use lossy compression for terrain, an incremental patcher wouldn't work very well either, because incremental changes in compressed binary files are not very efficient. In FS9 textures are all separate files, but still many will be affected by such change.
And, if we start distributing full installers and updates, it would become more complex and time consuming (and it will be worse with each subsequent release) for users to reinstall. Take Zurich as an example, if a user wanted to reinstall 1.3, he would have to install 1.0, then update to 1.1, then to 1.2, then to 1.3, instead of having just a file to keep track of, like we are doing now.
The very concept of a base product + updates, is the only method for developers without a Trial version: they don't have any other choice than using updaters, because they don't let you download the full product freely. Since this is not our case, offering a single installer it's the easiest solution (especially long term, when there will be more updates) for everyone.