Currently not all the shader parameters can be modulated by textures. For more flexibility & better materials these should be:
Some of this parameters are more important to be map-able (like exponent and transmission color), others less. But being able to do so would result in much more variety for basis shaders, without the need to use blend (which is slow and currently buggy in some cases). The blend shader cannot fully offer this funcionality for the parameters that can be set only by single values. For example; if you would want to create a multicolored glass with the blend shader, you would need to mix together a number of glass materials, which is unflexible, slow and messy (a deep hirarchy of blends might cause also other rendering problems).