With all these advantages, why not use adjustment layers for all your edits? The two reasons we consider reasonable are file size and file complexity.
Adjustment layers and file size
Adjustment layers use more RAM and hard drive space, both for scratch space and storage, than simply burning edits into a flat file. Adjustment layers themselves add very little to the RAM requirements because they contain almost no data, but painting on the Layer mask adds pixels that take up space.
Even if using an adjustment layer exceeds your available RAM, you may not see any slowdown because of the way Photoshop handles its image cache and scratch space. As Photoshop keeps getting smarter about the way it handles memory, it becomes more and more possible to do the seemingly impossible; so don't assume that you don't have enough horsepower to use adjustment layerstry it and see. (See Chapter 1, Building a Photoshop System, for more information on memory and scratch space, and Chapter 2, Essential Photoshop Tips and Tricks, for a more detailed discussion of the image cache.)
Adjustment layers and complexity
If you open a file you made a year ago, and it contains 20 layers with names like Curves 13 and Hue/Saturation 5, you may have to spend quite a bit of time remembering what all these layers did! Naming your layers, and organizing them into (informatively named) layer groups helps a lot.
All things considered, the disadvantages to using adjustment layers are minimal. We believe that most people will get better results faster using them. However, once you're happy with the edits, you may want to flatten the image to save storage space.