http://docs.gimp.org/en/gimp-channel-dialog.html

The is a Caution! box that tells the readers a tiny bit about the effects of deactivating channels, but this is IMO not enough to grasp what's going on.

Things that should be mentioned there:

1. Deactivating RGBA components does not change the way tools work

 A brush will continue to paint as usual, changing the pixels of the layer you're working on. The eraser will change the alpha value of the pixels you're working on, ect.

2. Activated RGBA components decide which part(s) of a tools action is applied to the layer 

Painting black in normal mode on a fully opaque layer does not make the layer go transparent even if the only channel you've activate is the Alpha one - the paintbrush does just not do this - and using the eraser will not change your image if the alpha channel is not active.


Conclusion: the RGBA channels are there to provde a way to changes (that would happen anyway) to the layer components, they are not designed to redirect changes to the activated components.