New tags should be added when they'll help categorize questions. New words for the same things that we already have tags for shouldn't get new tags.
If you see an unnecessary new tag, you should edit the tags on the question so that the question is tagged appropriately. (remove the unnecessary new tag)
I believe the tag does get cleaned up, but there may be a slight delay. If you look at the end of the tags list, you'll see that all of the "least popular" tags are either only used once, or are synonyms for something else.
Since the spider is an integral part of the crank/crankset (even the exceptions are swaged together, not bolted), and you really can't talk about a spider without talking about a crankset, I agree that the tag is redundant. It also seems likely that somebody may try to use that tag again.
I have used a moderator tool to merge spider into crankset, which cleaned up the (one) instance of "spider" and created a synonym, so that all future instances of spider will become crankset.
While I was in there, I saw that there was a proposed synonym of cranks to be crankset, and I used that same mod-access tool to change all instances of cranks to crankset and create the appropriate synonym.
If you have access to propose synonyms, you should do so. You can always use a custom flag or a meta post. If it seems possible to be a controversial tag change, a meta post will be better.