This is about skeletal animation.
Some people use uniforms to index into the the bone array.
I was wondering why not use an attribute for that instead?
warning : the code may have bugs as I quickly typed it
Using attributes (rather than uniforms) is pretty much the standard way to do skeletal animation. If you use uniforms you might just as well simply send the looked up values rather than the indices.
Back in the NV3x-4x era I was just able to squeeze in 8 index/weight pairs needed for the Doom3 models into 4 texcoord vectors. Tight fit uniform wise, with around 120 quaternion/vec4 bones or so, but it worked great.