Should I always have a primary key in my database tables?
Let's take the SO tagging. You can see the tag in any revision, its likely to be in a tag_rev table with the postID and revision number. Would I need a PK for that?
Also since it is in a rev table and not currently use the tags should be a blob of tagIDs instead of multiple entries of multiple post_id tagid pair?
You should strive to have a primary key in any non-trivial table where you're likely to want to access (or update or delete) individual records by that key. Primary keys can consist of multiple columns, and formally speaking, will be the shortest available superkey; that is, the shortest available group of columns which, together, uniquely identify any row.
I don't know what the Stack Overflow database schema looks like (and from some of the things I've read on Jeff's blog, I don't want to), but in the situation you describe, it's entirely possible there is a primary key across the post identifier, revision number and tag value; certainly, that would be the shortest (and only) superkey available.
With regards to your second point, while it may be reasonable to argue in favour of aggregating values in archive tables, it does go against the principle that each row/column intersection in a table ought to contain one single value. While it may slightly simplify development, there is no reason you can't keep to a normalised table with versioned metadata, even for something as trivial as tags.