To add a few remarks:
Tags are often used across weblogs, even across different sites (del.icio.us, flickr).
In EE, it really depends on what you store inside the installation. Categories and tags don’t make much difference if used for one weblog or a couple of weblogs sharing the same field group and categories. But if you have completly different setups not sharing the same structure and categories - say a recipes blog, a foto gallery and a cookbook “store” - tagging allows some sort of “related” entries across these borders: E.g. link from a book to receipes from that book or link from one barbeque recipe not only to other (same category) but to books on bbq and bbq photos as well (tags).
That said, tagging has some drawbacks as well: Because tags are used intuitive (and often different by different people) you may end up with one using “barbeque” as a tag for the above example, the other uses “bbq” or even “BBQ” ... which aren’t the same tags for most systems.
-Markus