Does the index in measure-global still serve a purpose?

[rpatters1]

The index member of measure-global has always seemed a bit of a one-off to me. It seems even more so now that all objects (including measure-global) have an id field.

It has seemed clear to me that index is not useful for finding the part measure for a global measure or vice-versa. That should strictly be done with the actual array index, I would think.

So what is index for? When should one prefer it over id and when should one prefer id?