Opening originally as a schema bug but maybe it's a spec bug for missing info. 

fx_sampler_common can legally be empty (as shown in the profile_GLES example). However, it's not clearthat this makes sense. Does it? If so, when? 

Here's part of the example:

<profile_GLES> 
  <newparam sid="gravel"> 
    <sampler2D/> 
  </newparam> 
  <newparam sid="grass"> 
    <sampler2D/> 
  </newparam> 
  <newparam sid="transition"> 
    <sampler2D/> 
  </newparam> 
  <technique sid="main"> 
    <pass sid="p0"> 
      <states> 
      <texture_pipeline> 
        <value> 
          <texcombiner> 
            <constant> 0.0f, 0.0f, 0.0f, 1.0f </constant> 
            <RGB operator="INTERPOLATE"> 
               <argument source="TEXTURE" operand="SRC_COLOR" sampler="gravel"/> 
              <argument source="TEXTURE" operand="SRC_COLOR" sampler="grass"/> 
              <argument source="TEXTURE" operand="SRC_ALPHA" 
sampler="transition"/> 
            </RGB> 
            <alpha operator="INTERPOLATE"> 
              <argument source="TEXTURE" operand="SRC_ALPHA" sampler="gravel"/> 
              <argument source="TEXTURE" operand="SRC_ALPHA" sampler="grass"/> 
              <argument source="TEXTURE" operand="SRC_ALPHA" 
sampler="transition"/>