In terms of blogs, I was pretty skeptical about tags when we first started using them... I much preferred the old Wordpress style of assigning posts to pre-defined categories. Obviously, this takes the freedom away from the user and forces posts to go into the admin team's categories.
When it comes to forums, I don't know what is best... could you give the user a list of tags from a drop-down menu, rather than making it a "free for all"?
Tagging and tag clouds are beneficial in identifying topics of interest. For things like images, photos, graphics, audio & video recording, tagging is imperative because there is no searchable text associated with the object. For forum threads or document areas, the case could be made that using a search function or creating a tag cloud based on words in the threads or documents could be used in lieu of tags although there are times when an implicit word is not used in the forum that might make a good tag. In this posting for instance, if these two words (taxonomy folksonomy) were not here, they might make good tags. Additionally, if I as an individual reader wanted to create a tag to mark something for my own purpose I could add a tag to a thread to place it into my own "folder". Google Reader is very clever in tagging RSS feeds this way. The end user creates "folders" to help organize feeds and those folder names in turn become tags for the content.
Over time more people will better understand the benefit to tagging. There might be some benefit to providing some awareness training to encourage tagging or you can just lead by example. Over time there will be more people using the feature.
While creating a tag cloud from content is highly desired, it would be a mistake to remove tagging capability.