Question

It seems to me that between glVertexAttribPointer (which specifies the attribute index to associate with a particular set of data from a VBO) and glBindAttribLocation (which specifies which program vertex attribute variable names the indices are assigned to, there’s not really a need to enable or disable the attribute indices themselves. What would be a situation that might require an index to be “disabled”?

Answer 1

What would be a situation that might require an index to be “disabled”?

If you want to set a constant vertex attribute value with glVertexAttrib, without passing a full blown array.

http://www.opengl.org/sdk/docs/man/xhtml/glVertexAttrib.xml