Why does the index need to be dropped?

Why does the index for which synonyms are to be created have to be removed and then recreated for this to work?

I see that the notification of this requirement was altered two months after the initial commit, and we've certainly seen that unless this is done, no changes to the elasticsearch results can be observed.

Given that synonyms might need to be added or altered long after an index has been created and indexed, what in this plugin makes this a requirement?