This is more a designing/reporting question than an ETL/ODI question ;).
The advantage of the dimension is that you can select or aggregate data per month, year, weekends, quarters, day of week, ...
If you only keep the date you will have to had all these complex formula's in logical columns or calculated items (assuming you use OBIEE). Performances will be impacted and code will be duplicated at many locations.
Hope it answers the question.