- creating a report (makes it easier and more intuitive) or
- creating the datamart (makes ETL or MDX calculations more straightforward without compromising the former) or
- user-experience benefits (query execution time)
If there is no tangible benefit to any of the above or, worse still, if any of them are compromised as a result of adding a dimension, it cannot be a good idea.
A good reason to create a dimension could be when two facts of different granularity need to be linked together, such as actual revenue and budget/plan/forecast (convenience and functionality). Another good reason is to normalize-out rapidly-changing attributes out of a fat, long and relatively static otherwise monster dimension (user experience and ETL benefit). Another one is to create a junk dimension when there are lots of unrelated low-granularity columns in a fact table (half a dozen of nominal categories with a few unique values that do not belong in any current dimensions)
Ultimately, if it is really a twilight zone and you are still sitting on the fence – just make it an attribute and promote it to a dimension once you will get a clear case for that: reduce your development time and business will love you for that alone 🙂
I hope this may helped someone decide. Please do comment and suggest your own!