The index is a power user feature. Its forced presence in Git effectively constitutes a usability barrier for new users. After all, a VCS is effectively a glorified abstraction for "save a file." Any barrier imposed between changing a file and committing it can get in the way and confuse people. The Git index does this.
Furthermore, the index is effectively a pseudo commit without a commit message. Any workflow using the index can be implemented in terms of actual commits itself.
I think because Git doesn't have strong usability in general and especially around history rewriting, many Git users feel that the index or an index equivalent is somehow a required feature of a VCS because Git's shortcomings give that illusion. However, if you use a VCS with better history rewriting (such as Mercurial with evolve), you'll likely come around to my opinion that the index can be jettisoned without meaningful loss of functionality or productivity.
Furthermore, the index is effectively a pseudo commit without a commit message. Any workflow using the index can be implemented in terms of actual commits itself.
I think because Git doesn't have strong usability in general and especially around history rewriting, many Git users feel that the index or an index equivalent is somehow a required feature of a VCS because Git's shortcomings give that illusion. However, if you use a VCS with better history rewriting (such as Mercurial with evolve), you'll likely come around to my opinion that the index can be jettisoned without meaningful loss of functionality or productivity.