Texture splatting - usually - is done by vertex painting, where each channel R-G-B-A is assigned as a different texture weight.
Due to the way shaders are executed, doesn't it mean that the fragment shaders will ALWAYS sample 4+ textures even if the weights are (1,0,0,0) or (0,1,0,0)?
Lets say some parts of the terrain are almost exclusively grass and the other part of the terrain is exclusively dirt, which means the times when more than one texture sample is necessary is very, very rare - on the transitions, but regardless 4 textures are sampled?
Or do GPUs perform some sort of optimization to minimize the impact of this?