I have noticed the same thing. We have a mod that automatically uses words from the thread title as tags. If it weren't for that I don't think we'd have any tags.
members should be permitted to add tags. The mod takes words only from the title, and not the OP content, which is a big drawback imo. There are definitely more words that can/should be added to the tag list.
The two forums I really wanted to add the tags to are using them. In order to submit a thread in those forums, they have a special form to fill out and I take certain words from that form and add them as tags. The mod that adds tags from the title would add too many tags that make no sense which is why I have to do my own thing there. I wish you could turn tagging on by forum because it can be very useful in some forums.
That is something I wondered why you can't do (after first trying to figure out how) when tags first came to vBulletin. I still think it is weird you can't - prefixes came at the same time and you can.
But you can. It is a setting in the acp. Every forum I do to, I can add tags to as a member, except for this one.
At the bottom near similar threads. I really do not like anything on the sides of a forum page. Home page is ok for side bar information but threads are hard enough to follow without distractions. We use tags as a filing system and only the moderators add tags, admin & super mods can create. Some threads are better than others and only the better threads are tagged and people can find those by clicking on a tag and browsing the threads that come up.