My informal answer is to say to use a taxonomy when you need a taxonomy. It may, very likely, be replaced in the future with CCK and you might want to do that now.
If you are simply looking for a drop down list than do that with a CCK field. But, that is different than a taxonomy system and there are a lot of add-ons to support taxonomy at this point.
Does that help?
But it may not be as important as it has been. I'll probably still need it because I'm trying to control access to site content by role with Taxonomy Access Control. But I'm getting a little concerned about the support status of that module. The D6 version is still officially in development with no updates for 3 and a half months. That seems to be one of the larger risks with Drupal - becoming dependent on modules that may or may not be well-supported for the life of your site.
Now that CCK is becoming "industrial-strength" for D6, is there still a valid reason to use taxonomy any longer? Perhaps I am missing the point (I frequently do...) but instead of using taxonomy, I can create a CCK field, which is a drop-down text field, and can populate that with the various categories etc. I can then use this field as a component of a view.
Isn't this fundamentally the same sort of thing that taxonomy is used for? When is taxonomy still an essential item?