Do we need a tag for each punctuation mark when there is already the tag punctuation?
At this point, I see no harm in having separate tags also. It seems very few questions have more than two or three tags anyway, and this could help make searches more effective.
From the use cases point of view, it seems unlikely someone would want a cloud of 'comma' related entries for polishing their skills with commas; or the dreaded colon v. semi-colon learning curve. However, as my poor skills of punctuation may show, I can imagine someone looking to improve their overall knowledge of punctuation and its usage.
As such, it seems to me that by consolidating specific marks in a 'punctuation' tag would be preferable to letting questions about individual marks hang out on the fringe with a useless tag. Obviously synonymizing (sp?) tags is another option, but if it's a black/white choice I find 'punctuation' to be more useful.
Yes, if the tag is to be any use it is very good to have a tag for "semicolon" so that people can find out what questions have been asked about this and don't repeat the same questions.