I have always read the tag label "similarities" as being the flip-side of the very popular tag label "differences." In my experience, "differences" comes up most often when a question focuses on distinguishing between two words that are close synonyms. But the opposite case, where a question focuses on two generally unlike words may have something in common, is quite rare. And since we have the tag label "synonyms" (as well as "alternatives" and—arguably—the dread "single-word request") to handle what amount to requests for alternatives to a given word, it seems not very useful to bring "similarities" to the party as well.
I suggest having the "similarities" tag automatically convert to "synonyms" going forward, after disposing of the questions that already include the tag—either by converting "similarities" to "synonyms" or by deleting the "similarities" label and (if possible) finding a different and more suitable replacement label. That would take care of instances where the tag currently appears in questions that ask about similarities in etymology or spelling.
I don't think that a brief surge in old posts with tag changes is a serious problem; but if most people feel otherwise, we (or sumelic) could convert, say, ten of them a day until the category is emptied.