When tags were first introduced to DEVONthink their visual representation was the same as the visual representation of groups (a folder), but a different color. Indeed, their logical similarity is very real; groups themselves can be treated as tags in DEVONthink.
By default now (but not when first introduced) groups are checked as excluded from tagging in Database Properties.
If you wish to apply identical group and tag properties to items (which could be viewed as redundant, although not necessarily so), unchecking the option to exclude groups from tagging does precisely that, in one step. Place documents into a group named, e.g., Physics, and that’s both their location and tag. Each document now has one group location and one tag.
Now suppose we replicate one of the documents to another group, e.g., Cosmology. Each instance (replicant) now has one group location. How many tags does it have? (This question can be answered by experiment.)
Groups, tags, aggregation by searches and smart groups (another form of aggregation by search) are ways of applying metadata to a collection of documents. We add metadata to collections of documents to help work with them for various purposes and workflows.
I could choose to create a database holding tens of thousands of documents without using group organization or tags. For my main database that I use for research and writing, my personal practice is to organize documents into groups, each of which defines a relationship (metadata) among documents placed within a group. I almost never tag new content when added (and do exclude groups from tagging almost always), but often find tags useful when I’m working on a project, to assign metadata to items for the purposes of that project. Those tags often result from aggregation of documents useful for the purpose of the project by searches or See Also suggestions–or perhaps because of my knowledge of the discipline and its literature. Because of the way I use such tags, which are related to the project of the moment, I usually remove most tags when a project has been completed. My experience is that they would probably be of little use or even be counterproductive for the purposes of the next project. Note that I don’t treat all my databases the same way, as they differ in types of content and the purposes they serve.
DEVONthink can accommodate a very wide range of workflows. If you find that for your purposes time and effort spent in assigning multiple tags to each item as it is added results in greater productivity in working with the database, do so.